Wetting pretreatment for enhanced damascene metal filling

ABSTRACT

Disclosed are pre-wetting apparatus designs and methods. These apparatus designs and methods are used to pre-wet a wafer prior to plating a metal on the surface of the wafer. Disclosed compositions of the pre-wetting fluid prevent corrosion of a seed layer on the wafer and also improve the filling rates of features on the wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S.application Ser. No. 14/593,676, filed Jan. 9, 2015, titled “WettingPretreatment for Enhanced Damascene Metal Filling” naming Mayer et al.as inventors, which is a divisional application of U.S. application Ser.No. 12/684,787, filed Jan. 8, 2010, issued as U.S. Pat. No. 8,962,085 onFeb. 24, 2015, titled “Wetting Pretreatment for Enhanced Damascene MetalFilling” naming Mayer et al. as inventors, which claims benefit under 35U.S.C. § 119(e) to U.S. Provisional Application No. 61/218,024, filedJun. 17, 2009, which are all incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

The embodiments disclosed herein relate to pre-wetting apparatus designsand methods.

More specifically, embodiments relate to pre-wetting apparatus designsand methods for pre-wetting a semiconductor wafer prior to depositingelectrically conductive materials on the wafer for integrated circuitmanufacturing.

BACKGROUND

Wetting is a property of a liquid/solid interface governed by adhesiveforces between the liquid and solid and cohesive forces in the liquid.Adhesive forces between the liquid and solid cause the liquid to spreadacross the solid surface. Cohesive forces in the liquid cause the liquidto minimize contact with the solid surface. The wetting of a solidsurface by a liquid is important in many industrial processes where aliquid interacts with a solid surface. Electroplating (a cathodicprocess), including electroplating in integrated circuit manufacturing,is one such industrial process. Wetting is also important in anodicprocesses, including eletroetching and electropolishing.

For example, in integrated circuit manufacturing, a conductive material,such as copper, is often deposited by electroplating onto a seed layerof metal deposited onto the wafer surface by a physical vapor deposition(PVD) or a chemical vapor deposition (CVD) method. Electroplating is amethod of choice for depositing metal into the vias and trenches of thewafer during damascene and dual damascene processing.

Damascene processing is a method for forming interconnections onintegrated circuits (ICs). It is especially suitable for manufacturingintegrated circuits, which employ copper as a conductive material.Damascene processing involves formation of inlaid metal lines intrenches and vias formed in a dielectric layer (inter-metal dielectric).In a typical damascene process, a pattern of trenches and vias is etchedin the dielectric layer of a semiconductor wafer substrate. Typically, athin layer of an adherent metal diffusion-barrier film such as tantalum,tantalum nitride, or a TaN/Ta bilayer is then deposited onto the wafersurface by a PVD method, followed by deposition of an electroplate-ablemetal seed layer (e.g., copper, nickel, cobalt, ruthenium, etc.) on topof the diffusion-barrier layer. The trenches and vias are thenelectrofilled with copper, and the surface of the wafer is planarized.

SUMMARY

Disclosed are pre-wetting apparatus designs and methods.

In one embodiment, a method of pre-wetting a wafer substrate prior toelectrolytically processing the wafer substrate is disclosed. A wafersubstrate having an exposed metal layer on at least a portion of itssurface is provided to a pre-wetting process chamber. Then, the pressurein the pre-wetting process chamber is reduced to a subatmosphericpressure. Then, the wafer substrate is contacted with a pre-wettingfluid at a subatmospheric pressure to form a wetting layer on the wafersubstrate.

In one embodiment, a method of electroplating a layer of copper on awafer substrate is disclosed. A wafer substrate having an exposed metallayer on at least a portion of its surface is provided to a pre-wettingprocess chamber. Then, the wafer substrate is contacted with apre-wetting fluid to form a layer of pre-wetting fluid on the wafersubstrate. The pre-wetting fluid includes water and copper ions and issubstantially free of plating additives. Then, the pre-wetted wafersubstrate is contacted with a plating solution to electroplate a layerof copper on the wafer substrate. The plating solution includes copperions and plating additives, with the concentration of copper ions in thepre-wetting fluid being greater than the concentration of copper ions inthe plating solution.

In one embodiment, a method of electroplating a layer of metal on awafer substrate is disclosed. A wafer substrate having an exposed metallayer on at least a portion of its surface is provided to a pre-wettingprocess chamber. Then, the wafer substrate is contacted with apre-wetting fluid to form a layer of pre-wetting fluid on the wafersubstrate. The pre-wetting fluid includes a water-miscible solvent.Then, the pre-wetted wafer substrate is contacted with a platingsolution including metal ions to electroplate a layer of metal on thewafer substrate.

In one embodiment, a method of electroplating a layer of metal on awafer substrate is disclosed. A wafer substrate having an exposed metallayer on at least a portion of its surface is provided to a pre-wettingprocess chamber. Then, the pressure in the pre-wetting process chamberis reduced to a subatmospheric pressure. Following the reduction ofpressure, the wafer substrate is contacted with a pre-wetting fluid at asubatmospheric pressure to form a layer of pre-wetting fluid on thewafer substrate. The pre-wetting fluid has a pH of between about 2 to 6.Then, the pre-wetted wafer substrate is contacted with a platingsolution including metal ions to electroplate a layer of metal on thewafer substrate. The plating solution has a pH of between about 2 to 6.The plating solution and the pre-wetting fluid also have differentcompositions.

In one embodiment, a method of electroplating a layer of metal on awafer substrate is disclosed. A wafer substrate having an exposed metallayer on at least a portion of its surface is provided to a pre-wettingprocess chamber. Then, the wafer substrate is contacted with apre-wetting fluid to form a layer of pre-wetting fluid on the wafersubstrate. The pre-wetting fluid includes a reducing agent to at leastpartially reduce surface oxide on the exposed metal layer. Then, thepre-wetted wafer substrate is contacted with a plating solutioncomprising metal ions to electroplate a layer of metal on the wafersubstrate.

In one embodiment, a method of electroplating a layer of metal on awafer substrate is disclosed. A wafer substrate having an exposed metallayer on at least a portion of its surface is provided to a pre-wettingprocess chamber. Then, the wafer substrate is contacted with apre-wetting fluid to form a layer of pre-wetting fluid on the wafersubstrate. The pre-wetting fluid includes a metal complexing agent to atleast partially remove surface oxide from the exposed metal layer. Thepre-wetting also has a pH of between about 4 to 12. Then, the pre-wettedwafer substrate is contacted with a plating solution comprising metalions to electroplate a layer of metal on the wafer substrate.

In another embodiment, a partially fabricated semiconductor devicestructure is disclosed. The partially fabricated semiconductor devicestructure includes at least one recessed feature, the recessed featurehaving a layer of metal substantially lining the feature. The recessedfeature includes a substantially gas-free pre-wetting fluid filling thefeature. The pre-wetting fluid includes an aqueous metal salt solutionsubstantially free from plating accelerators and levelers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a plot of bubble dissolution time versus feature size.

FIG. 2 depicts a plot of bubble dissolution time versus the dissolvedgas pressure.

FIG. 3 depicts a schematic layout of one embodiment of a pre-wettingapparatus.

FIG. 4 depicts an embodiment of a pre-wetting chamber.

FIG. 5 depicts an isometric view of an embodiment of a pre-wettingchamber.

FIG. 6 depicts an embodiment of a pre-wetting chamber configured for acondensation pre-wetting process.

FIG. 7 depicts an embodiment of a pre-wetting chamber configured for animmersion pre-wetting process.

FIG. 8 depicts another embodiment of a pre-wetting chamber configuredfor an immersion pre-wetting process.

FIG. 9 depicts an embodiment of an apparatus in which the pre-wettingprocess is performed in a plating cell.

FIG. 10 depicts an embodiment of an electroplating system.

FIGS. 11a and 11b are flow diagrams for embodiments of a pre-wettingprocess.

FIG. 12 is a flow diagram for an embodiment of an electroplating processfor electroplating a layer of metal on a wafer substrate.

FIG. 13 depicts a wafer substrate with a feature filled with pre-wettingfluid.

DETAILED DESCRIPTION

Reference will now be made to specific embodiments. Examples of thespecific embodiments are illustrated in the accompanying drawings. Whilethe invention will be described in conjunction with these specificembodiments, it will be understood that it is not intended to limit theinvention to such specific embodiments. On the contrary, it is intendedto cover alternatives, modifications, and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims. In the following description, numerous specific details are setforth in order to provide a thorough understanding of the presentinvention. The present invention may be practiced without some or all ofthese specific details. In other instances, well known processoperations have not been described in detail in order not tounnecessarily obscure the present invention.

Disclosed herein are apparatus designs and methods for waferpre-wetting, for modifying the conditions of wafer entry and waferprocessing during plating, and pre-wetting fluid compositions. Thepre-wetting process, in accordance with embodiments provided herein, canbe performed in the electroplating chamber or in a separate pre-wettingstation of a module which includes a pre-wetting station and anelectroplating station. In some embodiments pre-wetting andelectroplating are performed in separate apparatuses.

The substrate typically is a semiconductor wafer which has a layer ofconductive material residing thereon (e.g., a seed layer comprisingcopper or copper alloy). During electroplating, electrical connectionsare made to the conductive layer and the wafer substrate is negativelybiased, thereby serving as a cathode. The wafer is contacted with aplating solution containing a metal salt (e.g., copper sulfate, a copperalkylsulfonate, or a mixture of salts), which is reduced at the wafercathode resulting in metal deposition on the wafer. In many embodiments,the substrate contains one or more recessed features (e.g., vias and/ortrenches), which need to be filled by the electroplating process. Theplating solution, in addition to metal salts, may also contain an acid,and typically contains one or more additives such as halides (e.g.,chloride, bromide, etc.), accelerators, levelers, and suppressors, usedto modulate electrodeposition rates on various surfaces of thesubstrate.

The disclosed processes and associated apparatus designs areparticularly applicable and necessary for electrofilling wider (e.g.,typically greater than 5 μm) and deeper (e.g., typically greater than 10μm) damascene structures (vias), such as those commonly found inemerging copper through silicon via (TSV) electrofill structures.Through silicon via structures are further described in U.S. patentapplication Ser. No. 12/193,644, filed Aug. 18, 2008, now U.S. Pat. No.7,776,741, which is herein incorporated by reference. Gas bubbles,trapped or otherwise residing on the surface or within a feature, willinterfere with the field and feature plating process either by blockingthe feature surface with the non-conducting gas, or by creating animpediment to the free passage of current. The disclosed processes andassociated apparatus designs enable void-free copper electrofilling.

Electroplating and electrofill of TSV interconnections present a numberof challenges. These include long plating times due to very large and/ordeep structures, and formation of side wall voids due to seed layercorrosion reactions with plating electrolyte solutions and due toinsufficient coverage of lower sidewalls by PVD-deposited seed layer.Further, it is important to ensure that the interior of all recessedfeatures are filled with liquid and that there are no trapped gassesinside the features that prevents plating therein. It is alsoadvantageous to simultaneously maintain strong wall and field platinggrowth-suppression while removing plating resistance selectively at thebottom of the feature.

The pre-wetting apparatus designs and methods described herein aregenerally described with respect to electroplating (a cathodic process)a metal, specifically copper. However, the pre-wetting apparatus designsand methods described herein are generally applicable to allelectrolytic processes, including eletroetching and electropolishing,both of which are anodic processes.

Methods for forming liquid-filled bubble-free recessed features that areneeded for the plating process are described. Further, compositions ofpre-wetting fluids which minimize seed layer corrosion andsimultaneously increase plating rates are described.

INTRODUCTION

The concentration of dissolved gas at the bubble interface with theliquid is related to internal bubble pressure by Henry's law, one formof which can be expressed as:

C _(i) =x _(i) H _(i) P _(i)  (1)

where the subscript i depicts “inside” the bubble, C_(i) is theconcentration of a component of dissolved gas molecules in the liquidphase at the bubble interface (e.g., nitrogen, oxygen, etc., each inmoles/l), x_(i) is the mole fraction of that component in the gas phaseinside the bubble itself, H_(i) is the Henry's law constant, and P_(i)is the pressure inside the bubble. This equation can be written for eachmolecular component of the gas in a mixture of gases (e.g., one foroxygen, one for nitrogen, etc.). There is a similar expression for theconcentration of dissolved gas in the bulk solution, where the subscriptb is used to indicate the solution “bulk”, for example, with P equal toP_(b), signifying the gas phase pressure which would be in equilibriumwith a concentration of the species in the bulk, C_(b). Ignoring 2D and3D dispersion effects, and assuming that the diffusion of gas moleculesfrom within the bubble gas phase to the bubble/liquid surface is notrate limiting (so that an equilibrium condition between the dissolvedgas at the bubble interface and the concentration of gas inside thebubble is maintained), a useful approximation for the rate of gasdissolution from the bubble trapped inside a feature can be obtained,expressed as:

R=dV/dt=DH(x _(i) P _(i) −x _(b) P _(b))/h  (2)

where V is the bubble gas volume, t is time, D is the diffusioncoefficient of the gas in the solution, h is the distance from the topof the trapped bubble to the edge of the boundary layer thickness, whichresides at a distance 6 above the upper wafer plane, and the subscript bcorresponds to conditions in the bulk of the solution at the diffusionboundary layer interface. For a given chemical system at a fixedtemperature (constant Henry's law constant and diffusion coefficient),two factors can lead to relatively rapid bubble dissolution: 1) a largeconcentration difference/driving force (x_(i)P₁−x_(b)P_(b)); and 2) ashort diffusion distance h.

If the value of the driving force term H(x_(i)P₁-x_(b)P_(b)) is zero,the rate of dissolution is zero. Generally, this term is very small.Since the gas within the bubble typically comes from air inside a via ina wafer prior to the pre-wetting process, and the liquid is typicallysaturated with the same air prior to the pre-wetting process, the molefraction at the bubble interface and in the bulk solution will initiallybe the same as that of air (e.g., x=0.21 for oxygen, both in the bubbleand in the bulk solution). Therefore, for this situation and in general(i.e., unless other mechanisms are employed to enhance bubbledissolution), it is primarily the natural capillary difference inpressure within the bubble versus outside the bubble that results inbubble dissolution.

Trapped gas held within a small damascene feature (e.g., a via) canexhibit a very large internal pressure, due to strong internal capillaryforces. The total internal capillary pressure is proportional to thecontact angle and the surface tension, and inversely proportional toradius of curvature of the bubble,

P _(i) =P _(ext)+σ cos θ/r  (3)

where P_(i) is the total internal pressure within the bubble, P_(ext) isthe external pressure of the fluid (typically about 1 atmosphere), σ isthe liquid/gas surface tension, θ is the solid/liquid/gas contact angle,and r is the radius of curvature. Note that the radius of curvature rcannot significantly differ from the feature width, so one can oftensubstitute the radius of the via as an approximation for a bubble'sradius of curvature. With a small via, the total internal pressure (andhence the partial pressure of each component) can become very large,exceeding several atmospheres or more. These large internal pressuresthen drive a non-equilibrium condition with respect to the bulk of thesolution, and the bubble interface becomes significantly supersaturatedwith respect the amount of dissolved gas in the bulk of the solution atthe same pressure (i.e., the amount of dissolved gas at the bubble'sinterface exceeds the gas solubility in the liquid). This satisfies oneof the conditions for rapid bubble dissolution. For small vias, thesmall diffusion distance, “h”, also aids in a rapid dissolution rate.

In contrast, large vias with larger radius bubbles have both smallexcess internal pressure and much larger diffusion distances.Calculation/modeling of the time for complete bubble dissolution forvarious conditions (i.e., partial pressure of dissolved gas, rotationrate of wafer) as a function of the via depth for vias of a 3:1 aspectratio (depth to width) with the via initially 50% filled of gas atatmospheric pressure, are shown in FIG. 1. For all of processes shown inFIG. 1, σ=60 dyne/cm (e.g., a value for water), D=1.9E-5 cm²/sec (e.g.,a value for air in water), T=20° C., and Vi=50% of a via.

Vi is the initial volume of the bubble, under 1 atmosphere of pressure(i.e., only 50% of each via is filled with a bubble for generating theseplots). For the P_(ext)=0.2 cases, the pressure on the fluid is stillone atmosphere, but the partial pressure of dissolved gas in the bulk ofthe liquid is only equivalent to that in equilibrium with 0.2atmospheres of gas pressure. This condition could be achieved, forexample, by having a trapped bubble form by flooding the surface with adegassed fluid with a pressure of 0.2 atmospheres while the pressure ofgas over the fluid is 1 atmosphere. For the P_(ext)=3 cases, the amountof dissolved gas in the liquid is equal to that in equilibrium with 1atmosphere of pressure, but the pressure on the liquid and bubble isequal to 3 atmospheres of pressure. This condition could be achieved,for example, by having a trapped bubble form by flooding the surfacewith atmospheric-saturated liquid, and then applying an externalpressure over that via/liquid/wafer of 3 atmospheres. In this case, thebubble immediately shrinks in size to ⅓ its original volume.

Comparing curves A and F (non-degassed pre-wetting fluid, with theamount of gas equal to that in equilibrium with 1 atmosphere of air) tocurves B and C (a pre-wetting fluid degassed to a partial pressure equalto 0.2 of an atmosphere), the degassed solution cases have a lowerbubble dissolution time. Curves F and C are similar by comparison, butthe boundary layer thickness and dissolution time are greater becausethe wafer was rotated at a slower rate (12 rpm versus 90 rpm in A andB).

Curves A and F of FIG. 1 show that the time of dissolution of a bubbleinside a via where the solution is saturated with air changes more than5 orders of magnitude between a via 0.2 μm in size versus 50 μm in size.In small, submicron features, bubbles are unstable and dissolve quickly,but in larger features, bubbles will persist for very long times. Forexample, calculations indicate that a relatively large front end of theline structure 1 μm in diameter and 4 μm in depth completely filled withgas will have that gas completely dissolved in less than a 4 seconds. Incontrast, a 0.25 μm feature, 1 μm deep, is so unstable that it woulddissolve in less than 0.4 seconds, and smaller structures essentiallydissolve instantaneously. However, both of the favorable factors (i.e.,high internal pressure and short diffusion distances) are missing inlarge TSV scale structures. In contrast, calculations show that it couldtake over 2 hours for a 25 μm wide, 100 μm deep feature to dissolve.Even if this feature were only filled 10% with gas at its bottom, itwould still take 20 minutes or more for that gas to be removed.

Removing gas from the pre-wetting fluid reduces the time to dissolve atrapped bubble. In this case, the right term of the driving force(x_(b)P_(b) in equation 2) is diminished by stripping the gas from thesolution, for example by reducing the partial pressure of the gasexposed to the pre-wetting fluid in a degassing unit under partialvacuum (i.e., by driving down the magnitude of this product in the gasside of the degassing unit, gas is driven out of the fluid). The gas inthe trapped bubble is at approximately 1 atmosphere of pressure or more(when there is significant capillary pressure). At the bubble interface,the concentration of gas will be at near equilibrium with that of thesame 1 atmosphere or more or pressure, but in the solution, theconcentration as a whole, due to the degassing operation, is at a muchlower concentration. This creates a significant concentration drivingforce and degree of sub-saturation of the gas in the solution (chemical“capacity”) to enable rapid dissolving the bubble.

This procedure at first may appear to be appealing, but may suffer fromtwo limitations. First, for large deep vias, the diffusion distance forthe gas may still be a significant limiting factor. Second, since theamount of gas in the solution can never be less than zero, the magnitudeof the driving force for dissolution is limited to be no more thanapproximately Hx_(i)P (P=1 atmosphere). Comparing curves B and C to Aand F of FIG. 1, the rate of dissolution of large features (e.g., 50 μm)decreases by a more than one order of magnitude over the non-degassedgas, but the dissolution time is generally still unacceptably long(e.g., at least 5-10 minutes). Note that the rate of dissolution ofsmaller features are not significantly affected by the use of a degassedsolution, because the process is dominated by the large excess internalbubble pressure compared to the increase in 1 atmosphere of dissolve gasdriving force.

FIG. 2 shows the bubble dissolution time for various feature dimensions(at 90 rpm rotation, 60 dyne/cm), where the amount of dissolved gas isthe independent parameter. In each case, the bubble is initially 50% ofthe via size, and there is an external pressure of 1 atmosphere on thefluid and bubble, though the dissolved partial pressure changes as afunction of the x axis. To clarify, in FIG. 2, the concentration ofdissolved gas corresponds to the dissolved gas pressure on the x-axis,related by Henry's law. These partial pressure would be obtained, forexample, by degassing the contacting fluid to the extent of the x-axisparameter. Bubbles in smaller, less deep features dissolve more quickly,the rate aided by the large internal capillary pressure. Again reducingthe partial pressure for smaller features has less of a relative effecton reducing the dissolution time. For larger features (e.g., 50 m×150μm), there is diminishing benefit of reducing the partial pressure ofdissolved gas below 30 to 40% of the saturated condition. In all but thesmallest and most shallow features, the time of dissolution exceeds 100seconds. Feature depth is a significant limiting factor in all cases,with deep features having long dissolution times.

Apparatus

Generally, the apparatus designs and methods described herein avoid theformation of a bubble within a recessed feature (e.g., a via) on a wafersubstrate by first removing gas, primarily all non-condensable gasses(e.g., nitrogen and oxygen), from within the feature before pre-wettingthe surface and feature with a fluid. To accomplish this, the wafer withthe recessed features is placed in a vessel that is suitable for bothholding the wafer and removing the gas from the wafer surfaces (e.g., avacuum vessel). In addition to the vessel itself, a mechanism forremoving the gas (e.g., a line connected to a vacuum source such as apump) and a mechanism for depositing liquid onto the surface while thevacuum conditions are maintained, are needed.

Described herein are various apparatus designs for pre-wetting a waferprior to, or within a short time after, the initiation of a platingprocess, where bubbles and gas that might otherwise be trapped withinfeatures recessed in a surface are avoided. Embodiments of a pre-wettingapparatus include various elements. Typically, a pre-wetting apparatusincludes a pre-wetting fluid storage and return tank, including liquidmixing devices and liquid level controllers and sensors. In someembodiments, the apparatus includes a pre-wetting fluid degassing flowloop. Such a degassing flow loop includes a circulating pump,routing/diverting valves, a liquid degassing element, and a connectionbetween the liquid degassing element and the system vacuum pump (used topump down and apply vacuum to various liquid degassing elements on thetool and the pre-wetting chamber), in some embodiments. A pre-wettingapparatus also includes a pre-wetting chamber. A pre-wetting chamber, insome embodiments, includes a two position (open/closed) vacuum waferaccess door or lid for access to the chamber and a combined door or lidand splash shield that prevents liquid from hitting and subsequentlydropping from the upper walls or door onto the wafer surface. In someembodiments, inside the chamber is a wafer holder for supporting androtating the wafer within the chamber. In some embodiments, the chamberincludes an air-dome chamber-heater, used to prevent liquid condensationon the walls of the chamber that would otherwise reside above the waferand the vacuum wafer access door and potentially drip onto the wafer.Pre-wetting chambers typically include an inlet port for pre-wettingfluid to enter the chamber and to direct pre-wetting fluid to land onthe upper surface of the rotating wafer and an inlet line and chamberport for drawing and releasing vacuum on the chamber, the inlet linecontaining a particle filtration device and the inlet port containing aflow diffuser configured to disperse incoming gas flow and minimizechamber flow turbulence. In some embodiments, the chamber includesliquid level sensors for monitoring an empty/ready andoverflow/over-full condition. Pre-wetting chambers also typicallyinclude a drain for removing liquid from the chamber and directing thedrained fluid back to the storage tank.

Embodiments described herein overcome the deleterious effects of trappedbubbles, particular those bubbles which can be formed in larger vias ortrenches in a wafer, by: (1) avoiding trapping gas in the via duringpre-wetting altogether by removing substantially all of the atmosphericnon-condensable gasses above the wafer and from within the via, and thenpre-wetting the wafer with pre-wetting fluid; and/or (2) significantlyincreasing the rate a bubble will dissolve by applying a large externalpressure on the fluid, thereby driving the bubble to dissolve in thefluid by creating a large supersaturated condition at the bubbleinterface. In addition to these pre-treating and pre-plating measures,in some embodiments plating is performed in a plating solution that ismaintained in a degassed state, and in other embodiments, the platingsolution is degassed in the line just prior to being exposed to thewafer surface.

In some embodiments it is possible to perform pre-wetting within anelectroplating cell, where the pre-wetting fluid has the samecomposition as the plating solution. However, for a variety of reasons,including the hardware complexity of combining plating processes withvacuum processes, pre-wetting (including vacuum feature-backfilledpre-wetting) is often performed in a different cell, sub-cell, or modulethan the plating cell. When pre-wetting under vacuum is performed in adistinctly different area of the plating cell, or in a distinctlyseparate module from the plating cell, rather than in the platingsolution, the composition of the pre-wetting fluid can be selected. Thepre-wetting fluid may have the same, or very similar, composition asthat subsequently used for plating the wafer. The pre-wetting fluid mayinclude all the elements of the plating bath (e.g., the same solvent(s)and same dissolved metal ions, acids, cation, additives and halides, atthe same or very similar concentrations as in the plating solution).Such a pre-wetting fluid may work in some embodiments. Alternatively, inother embodiments, a pre-wetting fluid that is very different from theplating solution may be used. For example, in some embodiments, apre-wetting fluid of 1) water, 2) a fluid with a substantially highermetal ion concentration than that of the plating solution, 3) a fluidhaving either a lower, different combination of, or no dissolvedhalides, 4) a fluid substantially free of one, a few, or all of theplating additives, or 5) water-miscible solvents may be used aspre-wetting fluids. Such pre-wetting fluids are further describedherein.

A number of factors should be considered when selecting a pre-wettingfluid composition, including the possibilities of: a) corroding themetal layers on a wafer substrate before initiation of plating; b)inhibiting the plating process (i.e., slowing down or inhibitingaltogether the feature metal-filling process); c) the loss ofpre-wetting fluid to subsequent pre-wetting fluid reuse; and d) altering(by adding, diluting, or concentrating) various critical speciesconcentrations within the plating bath over time. The latter process mayalter the metal ion concentrations, halide concentrations, organicadditives, etc., in the plating bath. These effects can be quitesubstantial. Furthermore, when using a pre-wetting fluid of a differentcomposition than the plating bath, performing the pre-wetting process inthe same module without enabling suitable mechanisms of removing andrecovering excess entrained pre-wetting fluid that would be added to theplating solution would generally require mechanisms for mitigating,monitoring and/or otherwise correcting for plating solution modificationover time. On the other hand, the use of hardware and a process whereinthe pre-wetting operation is performed in a separate treatment station,module, vessel, or sub-vessel of the plating cell that allows forseparation and recovery of this fluid may be advantageous because it canavoid such issues. With this background, and in order to simplify thedescription of core concepts of embodiments, many embodiments aredescribed hereafter in the context of a separate pre-wetting “station”and a separate “plating station”, with the wafer being transferred fromthe former to the latter. However, while perhaps favorable in somecircumstances (e.g., for avoiding mixing of unlike liquids or for otherreasons), the aspect of embodiments related to the particular choice ofpre-wetting materials, the general fluid, and plating processingsequences are not intended to be so limited.

FIG. 3 depicts a schematic layout of one embodiment of a pre-wettingapparatus (i.e., chamber 301 and associated hardware). The chamber 301is connected to vacuum pump 303 though an outlet in the chamber andthrough a three-way valve connection 305. On the other side of thethree-way valve is degassing loop 306 that includes a pre-wetting fluidtank 307, a degassing device 309, and a pump 311 for circulating thepre-wetting fluid around the degassing loop. In another embodiment, thepre-wetting fluid feed line and the vacuum line are not connected exceptat the chamber, and each has its own valve (i.e., there is no three-wayvalve). In an alternative embodiment, the chamber has an inlet foradmitting pre-wetting fluid and an outlet adapted for connection with avacuum pump. The position of the pump 311 can after the degassingelement, if it is desired to drive the fluid into the chamber by thepump rather than by being sucked into the chamber by a pressuredifferential between the pre-wetting fluid tank 307 and the chamber 301.

In some embodiments, the area in the pre-wetting fluid holding tank 307is purged of gasses by applying a vacuum to the holding tank using avacuum pump (not shown) so that a minimum amount of dissolved gas isachieved. The rate or removal of the gas from the pre-wetting fluid canalso be increased by increasing the exposed surface of the fluid to thevacuum, for example, by having the fluid re-enter the chamber from thecirculation loop in a spray or through a spray column. In the embodimentof the system shown in FIG. 3, pre-wetting fluid is circulated thoughthe degassing device 309 (e.g., in some embodiments, a membrane contactdegasser) for removing one or more dissolved gases (e.g., both O₂ andN₂) from the pre-wetting fluid prior to pre-wetting. Examples ofcommercially available degassing devices include the Liquid-Cel™ fromMembrana of Charlotte, N.C. and the pHasor™ from Entegris of Chaska,Minn. The amount of dissolved gas can be monitored with an appropriatemeter (e.g., a commercial dissolved oxygen meter (not shown)). Theremoval of the dissolved gas prior to the pre-wetting fluid enteringchamber 301 can improve the pre-wetting process, as is described herein.After degassing the pre-wetting fluid, optionally, the valve 315 betweenthe vacuum side of the degassing chamber 309 and the vacuum pump 303 isclosed (this prevents gas initially in the chamber from becomingdissolved in the degassed pre-wetting fluid; in some embodiments,separate pumps can be used for these two functions).

Unlike conditions that exist when using an apparatus similarlyconfigured to that of FIG. 3, if the pre-wetting fluid is not degassedprior to exposing it to a wafer under vacuum, dissolved gas from thefluid can be released from the fluid as it enters the chamber. Thisresults in bubbles forming inside the vias. While not wanting to belimited by a particular model or theory, a via bottom is a location ofnegative curvature, and it is believed that this location is aparticularly susceptible to nucleating a bubble and releasing gas fromthe pre-wetting fluid. If this occurs, bubbles will be formed from thepre-wetting fluid containing dissolved gas because it is supersaturatedwith gas under the pre-wetting conditions (e.g., vacuum in the chamber).The bubbles so formed can remain there after the pre-wetting process,which in turn can inhibit plating there and lead to associated defects.Therefore, in some embodiments (including the embodiment shown in FIG.3), the pre-wetting fluid used in the pre-wetting process is a degassedpre-wetting fluid. In some embodiments, the degassed pre-wetting fluidmay be a plating solution, and the pre-wetting methods described hereinmay be performed in the same chamber as the plating chamber itself. If aseparate pre-wetting chamber and apparatus are employed, but thepre-wetting fluid is not degassed, then intermittent and unreliablefilling results may be observed. For example, when vias on a wafer arefilled with pre-wetting fluid (with the wafer under vacuum) withoutfirst degassing the pre-wetting fluid, it has been found thatapproximately 15% of the vias still have air bubbles in them (asindicated by the same percentage having post-plating voids, indicativeof trapped gas bubble therein). Thus, in some embodiments, it isimportant to perform pre-wetting under vacuum (i.e., at a subatmosphericpressure) and with a degassed fluid.

In contrast, the use of a degassed pre-wetting fluid in combination witha pre-wetting operation under vacuum (i.e., at subatmospheric pressure)leads, in some embodiments, to significantly fewer feature voids thanwhen pre-wetting under vacuum alone is employed. In specific embodimentsthat give good protection against forming voids, a combination of adegassed pre-wetting fluid with pre-wetting under vacuum is furthercombined with plating in a plating solution that is degassed. Theplating solution may be degassed only in the initial stages of plating(e.g., for only about the first 10 minutes of the plating process), orremain degassed for the entire plating process (e.g., if the platingtime is greater). Experiments performed under these conditions producedvias that were void free.

Returning to FIG. 3, after the pressure in chamber 301 has reached a lowvalue (i.e., a subatmospheric pressure), the three-way valve 305 to thevacuum pump location is switched to connect to the line from thedegassing loop 306, and the three-way valve 313 of the degasser loop isset to allow fluid to be directed into the vacuum chamber 301. In someembodiments, the subatmospheric pressure is about equal to that of theboiling pressure of the pre-wetting fluid at the temperature ofoperations, which for water at ambient temperature is about 20 torr. Inother embodiments, the subatmospheric pressure is about 50 torr. Infurther embodiments, the pressure of 50 torr is maintained whilepre-wetting the wafer substrate. In alternative embodiments, thepre-wetting system is configured to initiate introduction of thepre-wetting fluid into the chamber and onto the wafer substrate afterthe pressure in the chamber has been reduced to less than about 50 Torr.In embodiments where the pre-wetting fluid tank 307 is at atmosphericpressure, liquid is drawn into the chamber 301 by the pressuredifferential between the vacuum chamber and the pre-wetting fluid tank.

The pre-wetting fluid wets the device side of the wafer surface of awafer in the chamber 301. Needle valve 317 can be used to meter the flowof the pre-wetting fluid into chamber 301. Embodiments of chamber 301are described herein. Chamber 301, in some embodiments, is a pressurechamber configured to apply an external pressure to increase the rate ofbubble dissolution, as described herein. In further embodiments of apre-wetting apparatus, the pre-wetting apparatus includes a transfermechanism configured for transferring the wafer substrate from thepre-wetting chamber to an electroplating apparatus.

In some embodiments, the pre-wetting fluid is cooled prior to injectioninto the pre-wetting chamber (e.g., 0° C. for water, or −10° C. forsuitable electrolytes). In other embodiments, the degasser is configuredfor cooling the pre-wetting fluid to a temperature of less than about20° C. Other examples of methods for cooling the pre-wetting fluidinclude passing the fluid over a heat exchanger in the pre-wetting fluidholding tank or though a in-line cooler (both not shown in FIG. 3).Cooling the pre-wetting fluid reduces the partial vapor pressure of thesolvent of the pre-wetting fluid, which allows for greater appliedvacuum, for example, to the degassing device. Lowering the temperatureof the pre-wetting fluid can also be effective in increasing both thesurface tension and viscosity of the pre-wetting fluid, which tends tomake the phenomena of degassing device “blow through” or “weeping” lessprevalent. Weeping can be a particularly difficult problem when dealingwith salt containing pre-wetting fluids, because weeping salt ladenfluids tend to dry and destroy the pores of the degassing device. Usinglower temperature fluids reduces the tendency of salt laden electrolyteto evaporate and flow, avoiding this known source of degassing devicefailure. For example, the vapor pressure of water (with a small amountof salt) is about 2.7 torr at −10° C. versus 17.5 torr at 20° C. and 32torr at 30° C. With a 20 torr vacuum (yielding about 0.5 ppm dissolvedatmospheric gas) applied to a degassing device, a 30° C. pre-wettingfluid will literally boil and leave salts around the pores of thedegassing device, and a 20° C. pre-wetting fluid will evaporate rapidly.Very little degassing device salting occurs when using a −10° C.pre-wetting fluid, however. Thus, in general, more dissolved gas can beremoved more efficiently from a degassing device with a lowertemperature fluid. In some embodiments, the pre-wetting fluid is cooledto a temperature of less than 20° C., for example 0° C. or less, whileit is degassed and before it enters the processing chamber. Also,reducing the temperature of the pre-wetting fluid reduces the rate ofmetal corrosion in the pre-wetting system.

In some embodiments of a pre-wetting apparatus, the surface of a waferis wetted with a pre-wetting fluid followed by the application of anexternal pressure to the fluid. The wafer surface is first contactedwith the fluid using an appropriate mechanism, usually immersing thewafer in a pre-wetting fluid (described herein). In these embodiments,the pre-wetting chamber includes an inlet for admitting pre-wettingfluid and the chamber is configured for operating at a higher thanatmospheric pressure during or after pre-wetting. The application of anexternal pressure to the fluid facilitates the removal of bubbles. Insome embodiments, the pre-wetting fluid is preconditioned so that it issubstantially free not just of oxygen (e.g., to minimize corrosion ofthe metal on the wafer), but of all dissolved non-condensable gasses,such as nitrogen and carbon dioxide, prior to the pre-wetting of thesurface, to accelerate the dissolution rate of any trapped bubble in arecessed feature. Exposure of a wafer to deoxygenated processing fluidfor use in the treatment of a semiconductor wafer is described in U.S.Pat. Nos. 6,021,791 and 6,146,468, which are incorporated herein byreference.

After immersion of the wafer into a pre-wetting fluid or covering of thewafer with a pre-wetting fluid, the region around the wafer (e.g., apressure chamber) is closed and sealed, and an external pressure isapplied to the chamber and fluid. Pressure may be applied pneumatically(e.g., introducing high pressure gas into the chamber in the area overthe fluid), or hydraulically (e.g., with the chamber substantially freeof non-dissolved gas and using a hydraulic piston or other suitabledevice to apply external pressure to the fluid). As the pressure in thechamber increases, the bubble will decrease from its original size. Whenusing pneumatic (gas) pressure to compress the trapped bubble, it may beimportant to avoid dissolving substantial amounts of gas into thepre-wetting fluid, particularly in the vicinity of the bubble. In someembodiments, a stagnant, relatively thick layer of fluid, for example,greater than 1 cm in thickness, is used. In other embodiments, thepneumatic pressure is applied to the chamber though a long tube with asubstantial resistance for dissolution of gas from reaching theinterface so that the gas that contacts the liquid does so over arelatively small surface area and has a relatively long diffusion path,limiting the amount of gas that can dissolve in the fluid over a periodof time. However pressure is applied, the driving force for dissolutionof the trapped bubble will increase with applied pressure. For a largebubble without significant capillary pressure effects, the driving forcefor dissolution will be approximately equal to the product of theinitial mole fraction of the particular gas component in the bubble andthe difference in applied pressure to the chamber and the initialpartial pressure of dissolved gas in the fluid. This later quantity willvary depending on the extent of degassing performed on the pre-wettingfluid.

While pressure can be applied either pneumatically or hydraulically, inpre-wetting embodiments that are not immersion embodiments, but rathercoverage of a wafer with a thin layer of pre-wetting fluid, thepneumatically applied external pressure will potentially allow gas toredissolve quickly into a (e.g., degassed) thin layer of pre-wettingfluid. There is a competition between gas uptake from the externalpressurized gas source versus gas dissolution into the liquid from thebubble. Therefore, a relatively thick layer of pre-wetting fluid shouldbe used for non-immersion pre-wetting operations. Also, there are alimited number of practical mechanisms for applying hydrostatic pressureto the thin layer of pre-wetting fluid on a wafer. One possiblemechanism for doing so is to create a face-up wafer and a pre-wettingliquid fluid containing cup. In contrast, there is a much widertolerance with thick layers of pre-wetting fluid and the immersionpre-wetting method. This is because pressure can be transmitted to thebubble by a purely hydrostatic mechanism, and alternatively, applicationof pneumatic pressure will not quickly re-saturate the pre-wetting fluidaround a bubble in a via with gas because of the relatively longdiffusion distances involved.

When pressure is applied, with the gas partial pressure in the bubbleexceeding that in the pre-wetting fluid, the bubble will begin todissolve. Eventually the bubble will completely dissolve, the total timefor which depends on parameters such as its initial size, the appliedpressure, and the original depth of the bubble inside the feature. Afterthe bubble has completely dissolved, some time should generally beallowed to pass before the pressure is released, so that any excessdissolved gas (beyond that which will be soluble at 1 atmosphere) canequilibrate into the pre-wetting fluid as a whole. This avoids thepossibility of re-nucleating a bubble inside the feature. When this isprocedure is followed, the bubble will be removed from the feature andnot reform upon release of the excess external pressure.

Referring to FIG. 1, curves D and E (90 versus 12 rpm rotation in aplating bath respectively) are calculated for the rates of bubbledissolution as discussed above, but in this case a) with the amount ofinitial gas dissolved in the solution equal to that in equilibrium with1 atmosphere air (same as condition A, i.e., no degassing of thecontacting fluid) and b) with an external applied pressure of 3atmospheres. For this case, the total pressure of dissolved gas in thebulk fluid is equal to air at 1 atmosphere, and at the interface of thebubble, in equilibrium with 3 atmospheres of pressure. Comparing cases Aand F (no degassing and no pressurization) with cases B and C (degassingbut no pressurization) and cases D and E (no degassing but withpressurization) in FIG. 1, the pressurization of the fluid appears to bea good method in terms of achieving the shortest time of dissolution.Using a previously degassed pre-wetting fluid (0.2 atmospheres) incombination with a 3 atmosphere external fluid pressurization (a casenot shown in FIG. 1) will generally lead to a further 50% reduction indegassing time for large features (3−1=2 atmospheres driving forceversus 3−0=3 atmospheres driving force), according to calculations.

Note, however, that there is a potentially significant added advantageof using degassed fluid in this operation beyond just the reduction ingas removal time (which could be achieved, for example, by simplyincreasing the pressure to, say 4 atmospheres of pressure in this case).After the release of the externally applied pressure to the chamber, gasfrom the bubble and some of the gas from the external source (ifpneumatically driven) will have dissolved into the pre-wetting fluid. Asindicated above, unless one waits for equilibration (which can be arelatively slow process, taking several minutes or more), there is atendency to re-nucleate and re-form a bubble inside the via, since afterreleasing the pressure, the fluid (particularly inside the feature)still can contain gas at a concentration in excess to that soluble atambient conditions/pressures (i.e., in excess of that which would be inequilibrium with a pressure one atmosphere). In contrast, if the fluidis degassed before application of the externally applied pressure, thisequilibration time can be largely eliminated, since there is asubstantial excess capacity to absorb the gas from the bubble andthereby avoid re-nucleation and precipitation of the bubble.

Finally, depending on the wafer's orientation and the surface tensionbetween the bubble and the inner via surface, it is possible that theshrinking of the trapped bubble by external compressive pressures downto a size significantly less than the diameter of the via could allowthat bubble to detach itself from the wall and subsequently rise out ofthe via mouth due to its own buoyancy. Once the bubble has exited thevia, the pressure can be removed without the possibility of the bubblebeing trapped inside. The terminal rise velocity of a bubble less thanabout 0.5 mm rising in an infinite media (no wall effects) depends onits diameter a, the kinematic viscosity v, and the Reynolds number Re,and can shown to be approximately given by:

$\begin{matrix}{V = {{\frac{2}{9}\frac{{ga}^{2}}{v}\mspace{14mu} {for}\mspace{14mu} {Re}} < 1.0}} & (4) \\{V = {{\frac{1}{9}\frac{{ga}^{2}}{v}\mspace{14mu} {for}\mspace{14mu} 20} < {Re} < 100}} & (5)\end{matrix}$

where g is the acceleration of gravity and v is the pre-wetting fluidskinematic viscosity (fluid viscosity divided by fluid density).

The difference in the behavior of these cases (i.e, (4) and (5)) is thatfor low Re, convection is negligible and no wake is developed behind therising bubble, versus the irrotational case (i.e., when the Reynoldsnumber is high), where wake drag is considered and results in twice thedrag. The time it takes for a bubble to rise the depth of the via can becalculated as t (sec)=h/V, which, for example, for a 10 μm diameterbubble in a 100 μm deep via (0.01 cm) would be about just under onesecond. Typically a 100 μm deep via might have a μm diameter opening, sothe assumption of a bubble rising in an infinite media is not correct,as wall flow-slip effects will increase the time. It is recognized thatone could further speed up the process if an external body force wereapplied to the system in excess to or instead of gravity. For example, acentripetal force could be applied by spinning the wafer with the waferopening pointed towards the center of rotation, helping to drive thebubble inwards.

Equations 4 and 5 underestimate the actual bubble rise time when thebubble diameter is close to the size of the via. This underestimationbecomes a factor when the assumption of the bubble rising in an infinitemedia becomes fundamentally incorrect (i.e., for bubble diametersgreater than about ¼ the feature diameter size). The shear flow stressesbetween the movement of the rising bubble and the via wall begin todominate under such circumstances. Still, the conditions that satisfythe assumptions can be achieved simply by applying more pressure to thesystem (shrinking the bubble further), or by simply accounting for theexpected longer bubble rise/clearing time when the bubble diameter isclose to the via diameter.

Different designs of pre-wetting chambers are described herein. Oneembodiment of a pre-wetting chamber is shown in FIG. 4. In thisembodiment, the pre-wetting chamber is configured for delivering thepre-wetting fluid onto the wafer substrate in a liquid form. Thepre-wetting chamber may also be configured for spraying or streaming thepre-wetting fluid onto the wafer substrate for a period of time. In FIG.4, a wafer 401 is held face-up in pre-wetting chamber 403 with waferholder 402. In some embodiments, the wafer holder is configured to holdthe wafer substrate in substantially a horizontal orientation (i.e.,parallel to the Earth's horizon), during the pre-wetting process. Inother embodiments, the wafer holder is configured to hold the wafersubstrate in substantially a vertical orientation during the pre-wettingprocess.

In a typical operation, vacuum is first pulled on chamber 403 thoughvacuum port 409, which is connected to a vacuum system (not shown). Thisreduces the pressure in the chamber to a subatmospheric pressure. Aftermuch of the gas in the chamber is removed by the vacuum, pre-wettingfluid is delivered onto the wafer surface from the nozzle 405 or othermechanism. In some embodiments, the pre-wetting fluid is degassed priorto contacting the wafer surface, again, to avoid gas being released asthe pre-wetting fluid enters the vacuum environment. The wafer may berotated with motor 407 during the pre-wetting fluid delivery process toinsure complete wetting and exposure of the wafer. In some embodiments,the pre-wetting chamber is configured to deliver the pre-wetting fluidonto the wafer substrate. In some embodiment, the pre-wetting fluid isliquid. In some embodiment, the pre-wetting fluid (a liquid) firstcontacts the rotating wafer substrate within about 3 cm of the center ofthe wafer substrate. After pre-wetting, the wafer is spun at a low rpmwith motor 407 to remove entrained pre-wetting fluid, but leaving a thinlayer of fluid on the wafer surface. Excess pre-wetting fluid is drainedand exits the vacuum chamber through port 411. The wafer is thentransferred to a standard plating cell such as a Novellus clamshell cellfor plating with a thin layer of pre-wetting fluid retained by surfacetension on its surface and within its features.

FIG. 5 depicts an isometric view of an embodiment of a pre-wettingchamber suitable for performing pre-wetting process described herein.FIG. 5 is a detailed schematic of a pre-wetting chamber similar to theembodiment shown in FIG. 4. Pre-wetting chamber 501 includes a motor 503for rotating the wafer during processing is anchored below the chambervia the chuck to chamber base 504 by a motor-and-bearing supportingmember 505, both of which also create a fluid seal between the bearing507 and the underside of the chamber and the bearing. The bearing is acommercially available vacuum-pass-through central shaft rotary bearing.The motor is attached via a coupling 509 to a drive shaft 511 thatpasses though a vacuum isolating bearing to the chuck base 513. Thechuck has three arms (515 is one arm) to support the wafer (wafer notshown), confinement pins, and other alignment apparatus 517 asappropriate.

At the lower section of the chamber is a drain 519 for removing excesspre-wetting fluid that may accumulate there after it is applied to therotating wafer. The fluid is flung out toward the chamber walls anddrops to the chamber base. In some embodiments, a wafer peripheral“fluid defector shield” (not shown) is positioned approximately in theplane of the wafer to deflect fluid emanating from the wafer edgedownward before hitting the chamber wall. The deflector shield may bemoveable, or the wafer and wafer chuck plane may be adjusted byappropriate vertical moving mechanisms and seals. Also at the base ofthe chamber is a vacuum inlet and vacuum release line 521, housed in afluid protecting shield 523 is some embodiments. This shield helpsprevent surges of gasses from unnecessarily disturbing fluids within thechamber as well as minimizing the amount of liquid drawn into the vacuumline by isolating the two. While the vacuum line (and shield) can belocated in the upper section of the chamber, it is advantageous to drawvacuum from below the wafer so as to minimize the propensity of anyparticles falling onto the wafer and forming defects. This can occur ifparticles or other materials enter the chamber during back filling thechamber with a gas or from the ambient environment while the chamberdoor is open. To minimize particles and other materials from enteringthe chamber, the chamber is typically backfilled with aparticle-filtered inert gas such as nitrogen, carbon dioxide, or argon,and a slight positive pressure of clean particle free gas is feed to thechamber while the door is open. The backfill gas is typically filteredand the entering fluid enters a flow diffuser mounted on the wall of thechamber, so as to avoid a gas flow jet that might dry the wafer ordisturb any chamber contents unnecessarily.

In some embodiments, a pre-wetting fluid nozzle 525 is located above andto the side but not over the centrally located wafer and wafer chuck,oriented and configured to spray or stream fluid to reach the wafercentral regions. In other embodiments, the pre-wetting fluid nozzle isattached to a movable arm which can be positioned over the wafer. In theembodiment shown in FIG. 5, the chamber vacuum door 527 is located alongthe walls of the chamber, and configured to seal to the chamber itself.It can be moved away from the chamber as well as downwards (or upwards)so a wafer can enter the chamber freely, and then be repositioned to thesealing position after a wafer is placed onto the wafer holding chuck.The doors and other elements that potentially could hold entrained fluidshould be designed such that the fluid may not drip onto the wafer. Forexample, the door's retracted position and associated hardware may bepositioned below the plane that the wafer creates on insertion into thechamber, so as to avoid dripping fluid of otherwise contaminating thewafer during transit in or out of the chamber.

In some embodiments, the upper section of the chamber, particularly theareas above the plane where the wafer sits in the chuck and is extractedthrough the door, are heated above the temperature of the wafer that isto be pre-wetted. This includes both the areas that reside above thewafer (top surface or vacuum dome, not shown in FIG. 5) as well as theperipheral areas around the wafer. This heating is useful in avoidingliquid from dropping from the ceiling of the chamber onto the waferbefore vacuum conditions are established, potentially trapping an airbubble inside a via where the drop fell, circumventing the desiredprocess of putting a pre-wetting fluid onto the wafer only when air isfirst removed from the vias. Similarly, during the placement of a waferinto the chamber, liquid falling from the walls onto the wafer surfacewould have a similar effect. By heating the chamber walls, condensationon the walls and ceiling is avoided, as well as enabling the rapidevaporation of any stray droplets that might otherwise reach thosepositions, thereby keeping these areas dry.

While not shown in FIG. 5, in some embodiments a vertically moveable andautomatable splash shield is positioned peripheral to the wafer andchuck and inside the chamber. The splash shield can be moved upwardsduring the application of fluid or at other times as suited to minimizeand avoid, among other things, liquid from contacting the chamber dooror upper walls. Alternatively, the wafer chuck can be moved downwardsdeeper into the chamber and below the plane of the vacuum door afterwafer insertion, accomplishing the same purpose.

In other embodiments, rather than delivering a pre-wetting fluid to thewafer surface, the wafer is immersed in or otherwise covered with apre-wetting fluid (e.g., by condensation) while vacuum conditions aremaintained above the fluid and wafer. Since the creation of a vacuumwithin the chamber creates conditions where there is substantially nonon-condensable gas in the chamber, the pre-wetting fluid is not impededfrom entering a via. Put another way, the liquid does not need todisplace any gas located within a via during pre-wetting, since the gashas been removed in a separate operation (pulling vacuum) prior to thepre-wetting operation.

For example, in one embodiment, after vacuum is applied to thepre-wetting chamber, a condensable fluid vapor is created within thechamber or introduced to the chamber (e.g., vapors of water (e.g., lowpressure steam), methyl alcohol, dimethylcarbonate, diethylcarbonate,isopropyl alcohol, dimethyl sulfoxide, and dimethyl formamide, or otherliquid used as the subsequent plating electrolyte, easily dissolvable ina subsequent rinse, or soluble in the subsequent plating electrolyte).In embodiments where the wafer substrate has at least one recessedfeature and the pre-wetting chamber is configured to deliver pre-wettingfluid onto the wafer substrate in a gaseous from, the pre-wetting fluidcondenses to form a liquid film on the wafer surface that fills therecessed feature with the pre-wetting fluid. FIG. 6 depicts anembodiment of a pre-wetting chamber that is configured for such acondensation pre-wetting process. FIG. 6 shows chamber 601 that has amoveable vacuum lid (alternatively an access door) 609 allowing accessto the chamber, a line to a vacuum source 611, a vacuum release line613, and a condensable fluid inlet 615. Vacuum seal 617 seals the lowervacuum containment vessel 619 from the rest of the chamber. The wafer603 sits on a wafer cooling element (chiller) 605 that is part of thewafer holding fixture (chuck) 607. The wafer cooling element 605 reducesthe wafer substrate surface temperature to a temperature below thecondensation temperature of the pre-wetting fluid that flows into thechamber thorough inlet 615 as a vapor. In another embodiment, aftercreating a vacuum and removing the condensable gasses (e.g., air) fromthe chamber 601 with vacuum, water is simply heated and allowed tovaporize (i.e., boil) in the chamber, and allowed to condense on thesurfaces, including and preferentially on the cooler wafer 603, insidethe chamber. For example, in a chamber without vacuum seals 617, a smallamount of water in the lower section 619 of the chamber could be heatedand allowed to flash simultaneously while vacuum is pulled inside thechamber. The connection to the vacuum can be removed (closed) at somepoint during the process.

In another embodiment, the wafer substrate is immersed into a bath ofpre-wetting fluid for a period of time. FIG. 7 depicts an embodiment ofa pre-wetting chamber configured for such an immersion pre-wettingprocess. In FIG. 7, the wafer 701 is held in wafer holder 702 in achamber 703. Chamber 703 has an inlet 711 for admitting pre-wettingfluid. As shown, the wafer is held in the wafer holder face up, and heldby an appropriate mechanism that still allows fluid to reach wafer fromthe peripheral edge. Vacuum is pulled on chamber 703 though vacuum port707, which is connected to a vacuum system (not shown). Then, the waferis wetted with a pre-wetting fluid by, for example, either 1) the waferand wafer holder moving down into the pre-wetting fluid 713 or 2) thepre-wetting fluid level rising by fluid entering through inlet 711.During the pre-wetting process, the wafer may be slowly rotated withmotor 705. After the pre-wetting process, the liquid level is lowered,or the wafer raised, and the wafer is spun at low rpm with motor 705 toremove excess entrained fluid, leaving a thin pre-wetting fluid layer. Aflow of nitrogen gas through port 709 may also be used to dry thebackside of the wafer while the frontside of the wafer remains wetted.The wafer is then transferred to a standard clamshell for plating.

In other embodiments of the pre-wetting chamber shown in FIG. 7, thewafer can be held in a face down position. In some embodiments of apre-wetting apparatus with a pre-wetting chamber as shown the FIG. 7,the pre-wetting apparatus is configured to initiate immersion of thewafer into the pre-wetting fluid after the pressure in the chamber hasbeen reduced to less than about 50 Torr. The pre-wetting chamber 703shown in FIG. 7 can be used in embodiments in which an external pressureis applied to dissolve bubbles, as described herein. The chamber andother components would need to withstand internal pressures instead ofor in addition to vacuum.

FIG. 8 depicts another embodiment of a pre-wetting chamber configuredfor an immersion pre-wetting process. FIG. 8 shows pre-wetting chamber801, wafer 809, and fluid 813 or wafer holder 803 moving relative toeach other. In this embodiment, the chamber and wafer holder 803 can betilted for precise control of the pre-wetting front and complete liquidremoval from the chamber. Also, the gap between the wafer 809 and thebottom of the chamber is small. As in FIG. 7, pre-wetting fluid in FIG.8 may enter/exit though port 811 and a vacuum may be pulled on chamber801 though vacuum port 807, which is connected to a vacuum system (notshown). Excess entrained fluid may be removed from the wafer surface byspinning it at a low rpm with motor 805. The embodiment shown in FIG. 8is particularly useful when pre-wetting the wafer substrate surface witha high-cost pre-wetting fluid, or when it is otherwise desirable to usea minimal amount of pre-wetting fluid (e.g., so the level of dissolvedgas can be maintained at low levels). After pre-wetting, the wafer istransferred to a standard clamshell for plating. A similar design of anarrow-gap, tilted surface pre-wetting apparatus, but without amechanism for applying a vacuum during the pre-wetting operations, isdescribed in U.S. patent application Ser. No. 11/200,338, filed Aug. 9,2005, now U.S. Pat. No. 7,690,324, which is herein incorporated byreference.

The chamber shown in FIG. 8 can also be used in the embodiment in whichan external pressure is applied, as described above. In this embodiment,the chamber and other equipment is designed or modified to be able towithstand and maintain internal positive pressures.

An embodiment of an apparatus in which the pre-wetting process isperformed in a plating cell is shown in FIG. 9. Alternatively, this canalso be stated that the pre-wetting chamber is configured to bothpre-wet a wafer substrate and to electroplate a layer of metal on thepre-wetted wafer substrate. In FIG. 9, chamber 901 is a plating cell,with a vacuum sealing surface being a section of the cell wall 903.Wafer holding fixture 905 holds wafer 915. In the embodiment depicted inthis figure, the plating cell contains an ionically resistive ionicallypermeable high resistance virtual anode (HRVA) 907 and a separated anodechamber (SAC) region 909. One example of an HRVA containing apparatus isdescribed in U.S. patent application Ser. No. 12/291,356, filed Nov. 7,2008, now U.S. Pat. No. 8,308,931, which is incorporated herein byreference in relevant part. See also U.S. patent application Ser. No.11/506,054, filed Aug. 16, 2006, now U.S. Pat. No. 7,854,828, which isincorporated herein by reference in relevant part.

Initially wafer 915 is held above the plating solution 913 and thevacuum is drawn on the chamber through vacuum port 911. When vacuum isdrawn on the chamber, vacuum typically should be drawn on the backsideof the wafer through the wafer holding fixture so that the wafer doesnot fracture. Afterwards, the fluid level 913 is raised, wetting thewafer surface. In some embodiments, this fluid is pre-wetting fluid, andin other embodiments, this fluid is a plating solution. In someembodiments, the fluid is degassed prior to contacting the wafersurface. Since there is no gas in the chamber, the fact that the waferis face down does not lead to any trapped gas-containing bubbles belowthe surface or inside the vias. After the pre-wetting is complete, thevacuum can be released. Electroplating a metal (in some embodiments,copper) on wafer 915 can then begin. It is generally simpler(mechanically and processing conditions) to perform plating at ambientpressures, with or without wafer rotation. Alternatively, the vacuum canbe held throughout the electroplating process. Again, it is advantageousin this and other embodiments to have the fluid degassed prior toperforming the pre-wetting operations. Otherwise the fluid may releasedissolved gas, forming bubble inside the features or on the surface asgas is driven out of the liquid by the lower pressure.

A general description of a clamshell-type plating apparatus havingaspects suitable for use with embodiments described herein is describedin detail in U.S. Pat. Nos. 6,156,167 and 6,800,187, which areincorporated herein by reference for all purposes.

FIG. 10 depicts an embodiment of an electroplating system/module 1001for processing wafers. The particular tool layout shown contains twoseparate wafer handling robots, 1003 which moves a “dry” wafer from acassette stationed in a front FOUP (front opening unified pods) loader1005 to an aligner module/transfer station (not shown) and a transferchamber robot 1004. The aligner module ensures that the wafer isproperly aligned on a transfer chamber robot 1004 arm for precisedelivery to other chambers/modules of the system. In some embodiments,the aligner module both aligns the wafer azimuthally (so called “wafernotch aligning”) as well as in the vertical and horizontal planes to aparticular location (i.e., fixes the wafer's x, y, and z positionregistry).

The same or a different transfer chamber robot is used to feed the waferback from the back end “wet processing area” of the tool to the FOUPafter processing and drying is completed. A back end robot (not shown)may contain two or more arms, each with a single or multiple“end-effectors” to grip the wafer. Some “end-effectors” grip the waferat the bottom of the wafer with a vacuum “wand”, and others may hold thewafer only at is peripheral edge. In some embodiments, one robot waferhandling arm end-effectors is used only for handling a wafer that has awet surface, and the others reserved for handling only fully dry wafers,thereby minimizing contamination.

After a wafer enters the transfer station (containing transfer chamberrobot 1004), the wafer typically is fed to a pre-wetting chamber 1013(i.e., a pre-wetting apparatus is a station in a module, the modulefurther including an electroplating station configured to electroplatethe wafer with a metal, the metal being copper in some embodiments), thevarious embodiments of which are described herein. In other embodiments,system 1001 is configured for an anodic process. In such embodiment, themodule further includes a station configured for an anodic process, suchas electroetching or electropolishing.

The pre-wetting chamber 1013 is either configured to pre-wet a waferunder vacuum or to apply pressure to a wetted wafer, and in someembodiments, both. Using a pre-wetting chamber configured to pre-wet awafer under vacuum as an example, ambient air is removed from thechamber while the wafer is spun. Once vacuum is achieved, the deviceside of the wafer is exposed to degassed pre-wetting fluid (degassed inmodule 1015 with a degassing flow loop). After wetting is complete,excess fluid is removed, gas is reintroduced to the chamber toatmospheric pressure, and the chamber is opened to allow the wafer to beextracted by the robot or other transfer mechanism. In some embodiments,the transfer mechanism is configured to transfer the pre-wetted wafersubstrate from the pre-wetting station to an electroplating station inless than about one minute.

In some embodiments, the wafer is then placed into an aligner (notshown), such as a notch aligner. By passing though a high-accuracy notchaligner, accurate placement into a edge sealing plating cell, whichexcludes plating solution from the back and the very small device sideedge exclusion region (e.g., about 1 mm from the edge), is possible. Theplating cell may be specially designed to have a seal that traverses thenotch area. Plating and feature filling (i.e., a layer of metal iselectroplated on the wafer substrate) occurs in plating cell 1021, 1023,or 1025, (i.e., electroplating stations) and in some embodiments, theplating solution is a degassed solution. In some embodiments, the metalis copper. The electroplating stations are configured to immerse thewafer in a degassed plating electrolyte in the electroplating station.In some embodiments, an electroplating station is configured tocathodically polarize the wafer substrate before immersing the wafersubstrate in a degassed plating electrolyte. The plating solution can berecycled though a separate degassing loop different from a flow loopbetween a main plating bath and the plating sell, or by passing thoughthe degassing element in the same loop as the bath/plating cell loop,being degassed just prior to entering the plating cell.

After plating is completed the wafer is rinsed with water above theplating cell and spun to remove excess entrained fluid, the waferholding clamshell apparatus is opened releasing the edge seal andallowing wafer extraction. In some embodiments, the wafer is thenpicked-up from the plating cell and transported in to a metal removingisotropic etching module (ITE module) 1031. The ITE module is a wetetching module used to remove metal from the top of the wafer primarilyin the field region over the feature of the plated wafer, while leavingat least some metal inside the recessed features. Various designs ofsuitable equipment, etching processes, and etching chemical formulationsare described in U.S. Pat. Nos. 5,486,235, 7,189,649, 7,338,908,7,315,463 and U.S. patent application Ser. No. 11/602,128, filed Nov.20, 2006, now U.S. Pat. No. 8,158,532, Ser. No. 11/888,312, filed Jul.30, 2007, now U.S. Pat. No. 7,972,970, and Ser. No. 11/890,790, filedAug. 6, 2007, now U.S. Pat. No. 8,470,191, each incorporated herein byreference.

In addition, metal at the edge of the wafer is removed in the ITE module1031. Because a wafer is often held in an edge excluding clamshellapparatus, only a thin layer of seed metal exists at the outermostperiphery (the original seed layer) prior to the top side global etchingperformed here. Therefore, after processing here, it is common for theextreme edge of the wafer to be completely bare of metal, while the morecentral, non-plating protected and edge excluded region may have somemetal remaining (however, in other cases, the metal is removed from thatregion as well). This module can therefore perform both the global etchremoval of metal from the wafer as a whole, as well as remove metal fromthe outer periphery edge and outer peripheral bottom of the wafer, ofteneliminating the need to perform a more complex edge specific etchprocess, edge bevel removal (EBR), as described in, for example, U.S.Pat. No. 6,309,981, incorporated herein by reference.

In some embodiments, the progress of the etching process and thethickness distribution of the film is monitored in the etching module,for example, by measuring the cross wafer sheet resistance with an eddycurrent meter, or the reflection of an acoustic signal. Alternatively,the thickness after etching can be measured in the transfer station inthe dry state later in the process, and the process results monitored ormodified as appropriate to minimize any wafer-to-wafer performancedrift. After etching, the wafer can be either rinsed and dried in theetching module, or moved to a separate module, a wafer rinsing, cleaningand drying station 1041. There, any oxide film that may have formed inthe process sequence is removed or reduced (e.g., by applying a diluteacid solution to the surface), any residual chemical not removed by amore cursory rinse in the etching station is removed (both front andback of the wafer), and an edge bevel removal operation is performed asdesired (see, for example, U.S. Pat. No. 6,309,981). After rinsing thewafer with water, it is spun dry and then removed to a transfer station,where the front end robot re-deposits it into the wafer holder cassette.

One concern with the pre-wetting process is that in the time betweenpre-wetting and plating (i.e., after exposing the wafer to pre-wettingfluid while under vacuum in a pre-wetting chamber but before platingcommences), it is possible to have the wafer surface “de-wet”. Dewettingmay be described as a physical draining and coagulation of thepre-wetting fluid from the surface (i.e., rather than a drying of thesurface), leaving one section of the surface with a thicker film ofpre-wetting fluid, and another section with no pre-wetting fluidthereupon. This characteristic behavior is generally associated with ahighly hydrophobic surface with respect to the pre-wetting fluid. If thewetting layer pulls back or coagulates from a previously wetted surface,then the attributes of the pre-wetting process are lost. To avoid thisphenomenon, wetting agents can be added to the pre-wetting fluid toavoid the pooling of the fluid into puddles.

Surface oxides, surface contaminants, and other deposited materials onthe wafer surface that are exposed to air and humidity may be veryhydrophobic. For example, thin copper metal seed layered wafers exposedto air and water vapor will form a thin cuprous oxide layer, which ishydrophobic with respect to water. To avoid this potential problem, theoxide film can be removed in certain embodiments by, for example, byadding a small amount of acid (e.g., H₂SO₄, H₃PO₄) at a pH where theoxide is no longer stable, to the pre-wetting fluid used in thepre-wetting process. The acid will react with the oxide to form waterand the metal salt. The pre-wetting fluid can also contain small amountsof surface tension and contact angle lowering wetting agents (e.g.,surfactants, alcohols) which also avoid the phenomena. Pre-wetting fluidchemistry is discussed further herein.

In some embodiments where the pre-wetting operation is performed in aseparate chamber prior to plating, the pre-wetting fluid may contain asmall amount of metal ions, for example, to aid in avoiding theformation of bacteria in the system or modifying the oxide removalproperties. Alternatively, a metal oxide suitable reducing agent may beadded to the wetting solution, such as formaldehyde, glyoxylic acid ordimethyl-amine borane, or with a metal ion complexing additive (forcopper, examples might include ammonia, glycine, ethylene diamine).Further, the surface oxide or other contaminants can be removed bytreating the wafer in a reducing atmosphere (e.g., forming gas orhydrogen in argon), with or without heating, prior to the pre-wettingoperation. The temperature of the pre-wetting fluid and the wafersurface can also be increased or decreased from ambient conditions tooptimize the retention of fluid on the surface.

In some embodiments, operations in a pre-wetting chamber or apre-wetting chamber that is part of an electroplating system arecontrolled by a computer system. The computer includes a controllerincluding program instructions. The program instructions may includeinstructions to perform all of the operations needed to pre-wet a wafersubstrate. In one embodiment, the instructions are for reducing pressurein the process chamber to a subatmospheric pressure and subsequentlycontacting the wafer substrate with the pre-wetting fluid at asubatmospheric pressure to form a wetting layer on the substratesurface. The wafer substrate may be rotated at a first rotation rateduring delivery of a liquid pre-wetting fluid onto the wafer substrateat a subatmospheric pressure, the fluid delivery being performed forbetween about 10 to 120 seconds. Then, delivery of the pre-wetting fluidis stopped. After stopping the delivery of the pre-wetting fluid, thewafer substrate is rotated at a second rotation rate to remove excesssurface entrained pre-wetting fluid from the wafer substrate. In someembodiments, the vacuum in the process chamber is released after thedelivery of the pre-wetting fluid is stopped and prior to removal of theexcess entrained pre-wetting fluid. In alternative embodiments, thevacuum is released after removal of the excess entrained pre-wettingfluid. The wafer may be rotated at different rates in differentembodiments. In some embodiments, the first rotation rate duringdelivery of a liquid pre-wetting fluid onto the wafer substrate is lessthan about 300 rpm and the second rotation rate to remove excessentrained pre-wetting fluid from the wafer substrate is at least about300 rpm. In other embodiments, the first rotation rate is about 100 rpmor less, and the second rotation rate is at least about 500 rpm. In yetfurther embodiments, the pre-wetting apparatus is configured forremoving excess entrained pre-wetting fluid from the wafer substrate bya method selected from the group consisting of centrifugal spinning,air-knife drying, and wiping and the controller includes programinstructions for performing these operations.

Process/Method

In a general pre-wetting method for some embodiments disclosed herein, avacuum is first created in the environment around the wafer. Then, thewafer surface is sprayed with, streamed with, covered with, or immersedin sufficient (in some embodiments, degassed) pre-wetting fluid,eventually exposing the entire wafer to a sufficiently thick liquidlayer. The layer may not cover the entire surface at all times, untillater in the process. The wafer surface is then left immersed orotherwise exposed to a pre-wetting fluid layer for a period of time(e.g., by continuing to spray, stream, cover, or immerse the surfacewith additional fluid) until adsorption (or reaction) of any pre-wettingfluid constituents at the wafer surface have substantially reachedcompletion/equilibrium and a favorable/uniform wetting character(hydrophilicity, low contact angle) is achieved. After pre-wetting, thespraying, streaming, or covering of the wafer with pre-wetting fluid isstopped. In some embodiments, vacuum is released and then excessentrained fluid is removed from the (now) completely hydrophilic surface(e.g., by centrifugal spinning, air-knife drying, squeegee wiping,etc.), leaving a thin uniform adherent layer of pre-wetting fluid on thesurface. In other embodiments, excess entrained fluid is removed beforereleasing vacuum. Finally, the wafer is transferred to plating cell toplate the wafer.

Because there may be anywhere from a few seconds to over a minutebetween the time that entrained pre-wetting fluid is removed from thewafer surface to the initiation of metal deposition, it is importantthat the wafer is globally hydrophilic and remain completely coated withfluid over the entire surface. In the ensuing time, a hydrophobicsurface/fluid combination can lead to the fluid receding from, anduncovering a portion of, the wafer surface, for example, starting fromthe wafer edges. This de-wetting may cause the fluid to be drawn outfrom within any recessed features within the wafer substrate, possiblyleading to gas being trapped within the feature on immersion into theplating bath. Hydrophobic surfaces, particularly those that havecompletely de-wetted in some regions, have non-uniform fluid pre-wettinglayer thickness over the wafer substrate. In the case that thepre-wetting fluid in use has a different composition than the platingbath, the subsequent immersion of the pre-wetted wafer into the platingsolution will not allow for a uniformly wetted surface if thepre-wetting fluid has not wetted the wafer properly. The non-uniformlywetted wafer will cause the diffusion times and concentrations variouscomponents to be different across the wafer's surface due to thethickness of the wetted layer. This can lead to variation in featurefilling behavior or the creation of various wafer surface defects, suchas lines of entrapped bubbles, metal pits, metal thickness variations,or growth protrusions. Therefore, after the pre-wetting process, thepre-wetting fluid should create a uniform, small contact angle withrespect to the entire wafer surface, for example, a contact angle ofabout 45 degrees or less, if possible. When a lower contact angle ispossible, a very thin and adherent pre-wetting fluid layer can becreated.

It is often observed that the contact angles of a surface can changewith time, and that hydrophobic surfaces may become more hydrophilicover time when exposed to certain liquids. Certain wafer surfaces, suchas those coated with copper films by, for example, plasma vapordeposition, can exhibit a significant decrease in the liquid/surfacecontact angle with time upon continuously exposing the surface to thepre-wetting fluid. In particular, the continual exposure of such asurface, while under vacuum conditions, can lead to rapid and completetransformation of the surface from a generally de-wetted, hydrophobicstate, to a wetted, hydrophilic state.

Furthermore, this transformation, specifically when occurring undervacuum and with a degassed pre-wetting fluid, leads to particularlyfavorable low defectivity when combined with the subsequent platingoperation. While not wanting to be bound by any particular wetting modelor theory, if sufficient time to pass is allowed to pass (e.g., 5seconds to 1 minute) while the surface is immersed in, sprayed with,streamed with, covered with, or otherwise treated with asurface-tension-lowering pre-wetting fluid, the surface can undergo aconversion from a hydrophobic to a hydrophilic state. For example, byallowing time for low concentration constituents (e.g., wetting agents)time to adsorb to the wafer interface, or alternatively, time forspurious adsorbed species that reside on the surface (e.g., fromatmospheric exposure) to be desorbed from the surface, suitable stablewetting behavior can be obtained. Alternatively, agents in thepre-wetting fluid may react to slightly roughen the surface and/orremove thin surface layers such as surface oxides, nitrides, orcarbonates.

As a specific example, the transformation of a cuprous or cupric oxideladen surface, which tends to be inherently quite hydrophobic to water,to a hydrophilic metallic surface, is needed. By simple exposure toeither de-ionized (DI) water (which does not react with the oxide), thesurface may remain largely hydrophobic. Alternatively, by exposing thesurface to a slightly acidic oxide removing solution, such as DI watercontaining a small amount of either dissolved acid (e.g., sulfuric,methane sulfonic, or acetic acid, resulting in a pH of between about 2to 4), with or without dissolved metal ions and salts, a small amount ofmetal (e.g., copper) complexing agent (e.g., citrate, pH of betweenabout 3 to 6, glycine or ethylene diamine, pH of between about 6 to 12)or to a solution containing an appropriate metal oxide reducingagent/compound (e.g., formaldehyde, glycolic acid, dimethylamineborane), is effective in removing the surface oxide and transforming ahydrophobic interface to a hydrophilic interface.

Two examples of copper surface oxide removing reactions in a weak acidare:

CuO+2H⁺→Cu⁺²+H₂O, and  (6)

Cu₂O+2H⁺→2Cu⁺+H₂O→Cu⁺²+Cu+H₂O  (7)

A thin oxide surface layer of cuprous and cupric oxide on copper isformed almost immediately and continues to grow in thickness over time,simply as a result of exposure of the sputtered copper surface toatmospheric air, particularly moist air (i.e., air with humidity). Theoxide can be converted/removed by exposure to an appropriate removalagent (such as those listed herein), but it is important to considercomplete oxidation of the copper layer (also, e.g., within the feature).Subsequent removal of the metal oxide layer by use of an oxide removalprocess (as opposed to an oxide reducing process) may inhibit subsequentfilm growth for a fully oxidized copper layer. Also, the wettingconversion process (such as those listed herein) are chemical reactionshaving finite reaction rates. For example, exposing the wafer to theoxide removing pre-wetting fluid or plating bath will begin to form alayer of hydrophilic surface at the point of fluid contact. Areas withlonger exposure to the pre-wetting (e.g., oxide removing) fluid mayprevent other areas of the wafer from becoming wetted in the process.The hydrophilic areas that may be created can tend to channel fluid flowthereupon, preventing the wetting of other areas. One objective,therefore, is a modification of the contact angle, wetting properties,and general wetting process to enable the entire surface to becomeeventually uniformly covered with liquid, both macroscopically andmicroscopically.

By applying a degassed pre-wetting fluid to the surface whilesimultaneously maintaining a low pressure/vacuum atmosphere, theimpediment of simultaneously expanding, flushing, or otherwise removingtrapped gas from the surface is substantially eliminated, and so theimpediment of exposing areas of the wafer that are still hydrophobic dueto no or limited previous exposure to the pre-wetting fluid may bereduced. Considering the process without employing the vacuum andwetting combination, the various regions of the wafer surface will fallinto 5 wetted categories: 1) Hydrophobic Wetted: Covered with and wettedwith pre-wetting fluid but for an insufficient time, so it is stillhydrophobic; 2) Hydrophilic Wetted: Covered with and wetted withpre-wetting fluid for a sufficient time, so it is has becomehydrophilic; 3) Un-wetted: Hydrophobic, exposed to air, and neverexposed to pre-wetting fluid; 4) De-wetted: Previously wetted, buthaving become de-wetted, and again exposed to air; 5) Trapped Bubble:Containing a bubble containing trapped air at the surface and under alayer of pre-wetting fluid.

It is important to note that an area in state 3, 4, or 5 will notundergo any adsorption or chemical reaction, leading to the absence ofany hydrophobic-to-hydrophilic surface transformation unless and untilthat region later becomes wetted. Furthermore, areas around state 3 thatare in state 1 or 2 are wetted and are or will become hydrophilic,allowing fluid to flow freely and continuously over this surface andmaking the removal of the bubble or wetting of adjacent surfacesconsiderably more difficult. Also, a currently hydrophobic surfaceregion, having previously been exposed to pre-wetting fluid, mayrepeatedly go between the states of liquid-coverage-free and covered buthydrophobic. The process continues converting between these states asfluid wicks away to adjacent hydrophilic area, oscillating back andforth from state 1 to state 3 multiple times, until eventually it eitherit i) changes to state 2 and becomes hydrophilic and wetted, thereafterstays in state 2, or ii) become surrounded by areas that are morewetting, encapsulates a bubble, and transforms to state 4.

The above processes, performed under atmospheric conditions (i.e., inair), should be contrasted with processes performed under vacuum (andwith degassed pre-wetting fluid). In these processes, there are onlythree wetted categories that exist: 1) Wetted: Covered and wetted withpre-wetting fluid; 2) Un-wetted: Exposed to vacuum and never exposed topre-wetting fluid; 3) De-wetted: Previously wetted, but having becomede-wetted and re-exposed to vacuum.

A pre-wetting process performed under vacuum ensures that, as long as aparticular part of the wafer has been exposed to pre-wetting fluid(state 1) for sufficient time, the particular part of the wafer willeventually become hydrophilic. Unlike a pre-wetting process performed inatmosphere, a high fluid velocity pre-wetting fluid stream is notrequired to “flush away” trapped bubbles. Furthermore, bubble flushingis not 100% effective, and will often lead to bubble fragmentation,leaving a large number of smaller, hard to remove bubble behind. Hence,pre-wetting under vacuum is a much more reliable low defect process oversimply spraying, covering, or immersing the wafer into a pre-wettingfluid under atmosphere. Other factors that favor pre-wetting undervacuum are that a) surface energies of the vacuum/liquid/metal interfaceare different and the contact angle is often lower than theair/liquid/metal interface, b) metal oxide/nitride/carbonate reformationis avoided, and c) using degassed fluid prevents the possibility of gasprecipitating out of the fluid, for example, as a result of a spurioustemperature or pressure change at some points at the liquid-waferinterface.

FIG. 11a is a flow diagram for a general embodiment of a pre-wettingprocess (1100). A wafer substrate having an exposed metal layer on atleast a portion of its surface is provided to a pre-wetting processchamber (1105). The pressure in the process chamber is then reduced to asubatmospheric pressure (1110). The wafer substrate is then contactedwith a pre-wetting fluid at a subatmospheric pressure to form a wettinglayer on the wafer substrate surface (1115). Such a pre-wetting processcan be performed in the pre-wetting apparatus designs described herein.

The wafer substrate has different features in different embodiments. Thewafer substrate may have at least one recessed feature. The recessedfeature may be a damascene feature, which are formed by damascenepatterning processes. A damascene plating process is a process in whicha recess in a dielectric layer of a semiconductor wafer formed by adamascene patterning process is filled with a metal film. A recessedfeature may also be a though-mask feature.

In some embodiments, the pre-wetting fluid is substantially free ofdissolved gases. In some embodiments, one or more dissolved gases areremoved from the pre-wetting fluid prior to contacting the wafer withthe pre-wetting fluid. To aid in removal of dissolved gases, in someembodiments, the pre-wetting fluid is cooled to less than about 20° C.during removal of the gases. To remove gases from a pre-wetting fluid toobtain, in some instances, a pre-wetting fluid that is substantiallyfree of dissolved gases, a pre-wetting fluid treatment tank has thepre-wetting fluid circulating for a specific time period (typically ½ anhour, depending on the capabilities and capacity of the degasser) thougha degassing loop before contacting the wafer substrate with pre-wettingfluid. This is discussed herein with respect to FIG. 3. Typically thisimplies that fluid is flowing through the loop while the vacuum pump ison and at vacuum, and the valve connecting the degasser and to thepre-wetting tank to the pump is open. This ensures that the pre-wettingfluid that is subsequently applied to the wafer surface is substantiallyfree of dissolved gases. Measurements of a system so designed showsresidual levels of dissolved oxygen reaching as little as about 1-2% orless of that saturated with oxygen from air.

Furthermore, dome and wall heaters on the process chamber may be turnedon, set to a temperature of about 10° C., and in some instances about20° C. or greater, than that of the pre-wetting fluid temperature. Forexample, if the fluid temperature is about 20° C., a wall temperature ofabout 40 to 50° C. is appropriate. Dome and wall heaters avoidcondensation on the surfaces and the potential for liquid dropletsfalling onto the exposed surface prior to pre-wetting under vacuum. Apurge of the chamber surfaces can be accomplished by bringing thechamber to vacuum with the door closed and the walls at the targetheated temperature. For example, without a wafer present in the chamberand the walls heated, the chamber is brought to vacuum and remains atvacuum for about 10 minutes or more, so as to remove any liquid whichmight have accumulated on the chamber ceiling and upper walls. Thevacuum can be removed by backfilling with, for example, clean drynitrogen. This procedure removes any possible condensate from thechamber walls and minimizes the formation of gas born particles. Afterconfirming that a) all chamber fluid level sensors are at appropriatevalues (e.g., tank full, chamber empty), b) the heater is on, and c) thevacuum is ready for processing, the pre-wetting chamber process door canbe opened and the door shield (if so equipped) dropped. Next, a wafer isplaced into the chuck and the robot arm is retracted, the vacuum door isclosed, and the liquid splash shield is raised or the wafer loweredbelow the shield (if so equipped).

A target level of vacuum for the pre-wetting process in some embodimentsis between about 10 and 100 torr, for example about 40 torr. In someembodiments, the vacuum (i.e., subatmospheric pressure) is about 50torr. In some embodiments, after pump down is complete, the vacuum linecan be closed, while in other embodiments, the pump continues to pull avacuum while pre-wetting fluid is injected into the chamber and onto thewafer.

In some embodiments, a liquid pre-wetting fluid is delivered onto thewafer substrate surface. This may entail immersing the wafer substratein the pre-wetting fluid. Alternatively, this may entail spraying orcovering the wafer substrate with the pre-wetting fluid. In otherembodiments, contacting the wafer substrate with a pre-wetting fluid isperformed by delivering a gaseous pre-wetting fluid onto the wafersubstrate. The gaseous fluid is allowed to condense and form the wettinglayer on the wafer substrate. In these embodiments, the temperature ofthe wafer substrate may be reduced below the condensation temperature ofthe pre-wetting fluid before exposing the wafer substrate to thepre-wetting fluid.

In some embodiments, the wafer may be rotated while a liquid pre-wettingfluid is delivered onto the wafer substrate surface. In someembodiments, the wafer substrate is rotated at a rate of between about10 rpm to 300 rpm. In further embodiments, the wafer substrate isrotated at a rate of between about 10 rpm to 100 rpm. In otherembodiments, the wafer substrate is spun at speed of from about 100 to400 rpm, for example at about 300 rpm. In some cases a higher rotationrate (e.g., about 400 to 800 rpm), or a cycling of rotation rate, may beused for a short time (about 2 to 10 seconds) where overcoming fluidwetting resistance of a highly hydrophobic wafers is an issue. Chamberpump down may be initiated before of after wafer rotation is started.

In embodiments where a liquid pre-wetting fluid in used, flow of thepre-wetting fluid is initiated into the chamber and onto wafer surface.A typical flow rate of between about 0.5 and 2 lpm, for example, about0.8 lpm, is used, for between about 3 seconds and 1 minute or more, forexample, for about 20 seconds, depending on the necessary time toachieve full wetting of a particular surface, rotation rate of thewafer, and the wetting properties of the fluid. In some embodiments, thepre-wetting fluid is contacted with the wafer substrate from about 10second to 120 seconds. After the wetting process is complete, thepre-wetting fluid flow is stopped, for example, by closing a pre-wettingfluid flow valve.

Next, the chamber is brought to an atmospheric pressure. In someembodiments, the chamber is brought to an atmospheric pressure with anoxygen-free gas, e.g., dry nitrogen.

In some embodiments, excess pre-wetting fluid is removed from thesubstrate surface. This can be done before or after bringing the chamberto an atmospheric pressure. In some embodiments, excess pre-wettingfluid is removed from the wafer substrate surface by rotating the wafersubstrate. The wafer substrate rotation rate is increased to a valuewhere excess entrained fluid can be removed from the wafer substratesurface, but a thin layer of liquid remains. The wafer substrate may berotated from about 300 rpm to 1000 rpm during removal of the excesspre-wetting fluid. The wafer substrate may be rotated less than about 20seconds during removal of the excess pre-wetting fluid. In otherembodiment, the wafer substrate rotation rate is increased to betweenabout 250 and 800 rpm for between about 5 and 60 seconds, while avoidingthe complete drying of the pre-wetting fluid. While the rotation processgenerally can be initiated prior to the release of vacuum, by performingthis step after the release of vacuum, it is believed that the potentialfor the wafer drying is reduced, because the evaporative drying from athin layer and the possibility of creating a dry surface at some pointon the wafer may be less.

After removing the excess entrained fluid from the wafer substratesurface, the wafer substrate rotation is stopped, the splash shieldlowered and/or the wafer substrate raised (if so equipped), the vacuumdoor opened, and the wafer removed from the chamber and placed in anelectroplating chamber. In some embodiments, the pre-wetted wafersubstrate is exposed to an environment outside of the chamber and theelectroplating chamber for less than about one minute. In otherembodiments, the pre-wetted wafer substrate has a wetting layer having athickness of between about 50 to 500 μm immediately prior toelectroplating when it is transferred to the electroplating chamber.After the wafer substrate is in the electroplating chamber, the wafersubstrate is electroplated using a degassed plating solution, in someembodiments. In some embodiments, the pre-wetted wafer substrate iscathodically polarized with respect to a plating solution beforecontacting the wafer substrate with the plating solution. Thepre-wetting process chamber and the electroplating chamber may bedistinct stations of one apparatus module. In other embodiments, thewafer substrate is electroplated in the same chamber than was used forpre-wetting. In these embodiments, the electroplating may be performedusing a degassed plating solution.

In alternative embodiments, after removing the pre-wetted wafersubstrate from the pre-wetting process chamber, the pre-wetted wafersubstrate is transferred to a chamber configured to perform an anodicprocess such as electroetching and electropolishing.

FIG. 11b is a flow diagram for another embodiment of a pre-wettingprocess (1150). A wafer substrate having an exposed metal layer on atleast a portion of its surface is provided to a pre-wetting processchamber (1155). The pressure in the process chamber is then reduced to asubatmospheric pressure (1160). The wafer substrate is then contactedwith a pre-wetting fluid at a subatmospheric pressure (1165). Thepressure in the process chamber is then increased to facilitate theremoval of bubbles (1170). Such a pre-wetting process can be performedin pre-wetting apparatus designs described herein.

The apparatus designs and methods described herein may be used topre-wet a partially fabricated semiconductor device structure. In someembodiments, a pre-wet partially fabricated semiconductor devicestructure includes at least one recessed feature. The recessed featurehas a layer of metal lining the feature. The recessed feature alsoincludes a substantially gas-free pre-wetting fluid filling the feature,the pre-wetting fluid comprising an aqueous metal salt solutionsubstantially free from plating accelerators and levelers.

Different combinations of pre-wetting fluid compositions and platingsolution compositions can be used in a pre-wetting process combined withan electroplating process, as described herein. FIG. 12 is a flowdiagram for an embodiment of an electroplating process 1200 forelectroplating a layer of copper on a wafer substrate. A wafer substratehaving an exposed metal layer on at least a portion of its surface isprovided to a pre-wetting process chamber (1205). The wafer is thencontacted with a pre-wetting fluid to form a layer of pre-wetting fluidon the wafer substrate (1210). The pre-wetted wafer is then contactedwith a plating solution that includes metal ions to electroplate a layerof metal on the wafer substrate (1215).

The apparatus designs and methods described herein are useful in variousother liquid semiconductor processes and circumstances, beyondelectroplating/feature filling, where bubbles or trapped gasses within ahigh aspect ratio feature may pose a problem.

All operations described herein, including the various wetting,pre-wetting, degassing, alignment, transfer, and plating operations, maybe configured or programmed in one or more controllers provided on orotherwise in communication with the described modules and systems. Anycombination or sequence of such operations, as described herein, may beprogrammed or configured as such using such controller(s). Firmware,software macros, application specific integrated circuits, shareware,and the like may be used to implement the controller instructions.

Chemistry of the Pre-Wetting Fluid

By properly controlling the chemistry of the pre-wetting fluid, furtherbenefits of the pre-wetting process described herein may be realized,including a 50% or greater reduction in the filling time of a featurewith a metal. Furthermore, the feature filling process may startsignificantly more rapidly, reflected by the fact that under similarconditions (i.e., the same conditions, with the exception of thecomposition of the pre-wetting fluid), the amount of metal depositedselectively at the bottom of the feature in the same time is muchgreater. With particular organic and inorganic additive combinations tothe pre-wetting fluid, the pre-wetting process allows for an excellentside wall and field (field refers to wafer substrate regions that areflat and outside of features) metal growth selectivity versus bottom ofa feature metal growth selectivity, allowing for high rate selectivedeposition with greater than an order of magnitude relative platingrate/growth at the feature bottom versus the upper side walls and field.The selectivity achieved by controlling the chemistry of the pre-wettingfluid allows for bottom-up, often plug-fill, growth and the ability torapidly fill, without voiding, high aspect ratio features.

Historically, a number of different plating bath solutions used todeposit copper have been used to meet various needs/goals. Coppersulfate and copper methane sulphonate are the most commonly used metalsalts for electroplating copper, particularly in the integrated circuitindustry. The acid copper fluoroborate bath (mixture of copper andfluoroboric acid with boric acid), with its high solubility of copperand potential for high deposition rates, is also used, but has largelyfallen out of favor and replaced by the methane sulphonate system (whichalso has high copper solubility), at least in part because of thetendency for the BF₄ ⁻ anion to decompose and form hazardous HF.Alkaline copper cyanide and copper pyrophosphate baths have also beenwidely used, with cyanide baths having generally good platingperformance, but have fallen out of favor for toxicity and stabilityreasons.

While the scope of this description is not limited to electroplating ofs specific metal, or to specific plating solution and pre-wetting fluidcombinations described in the examples, electroplating of copper inplating solution baths containing copper sulfate and/or coppermethanesulphonate will be used as an illustration of specificembodiments. It is understood that embodiments disclosed herein can beemployed for deposition of metals other than copper, such as nickel,iron, gold, silver, tin, lead, zinc, as well as copper and other metalco-deposited alloys (e.g., various solders such as lead-tin andsilver-tin, or magnetic alloy materials containing iron, cobalt, andnickel). It is also understood that in copper electroplating, a varietyof other salts beyond copper sulfate and copper methanesulphonate may beused.

Copper sulfate and methanesulphonic acid plating bath solutionstypically contain three or more materials (so called plating“additives”) in small concentrations (10 ppb to approximately 1000 ppm)that affect the surface electrodeposition reactions. Typically,additives include accelerators (mercapto containing species, forexample; also referred to as brighteners), suppressors (typicallypolymers such as polyethylene glycol, for example; also referred to ascarriers), levelers, and halides (e.g., chloride ion and bromide ion),each having a unique and beneficial role in creating a copper film withdesired micro- and macro-characteristics.

The pre-wetting fluid and plating solution compositions described hereinmay be used with any of the apparatus designs or methods. For example,the pre-wetting fluid and plating solution compositions are able to beused with the methods described in FIGS. 11a, 11b , and 12.

There are several different categories of process interactions whichshould be considered in selecting the optimal pre-wetting fluid for awafer substrate. These various issues are discussed herein, along withthe presumed or measured examples of their effect on feature filling.

One consideration is that the surface tension of the pre-wetting fluidshould be sufficiently compatible with the wafer substrate surface(e.g., hydrophilic) so that the entire surface remains covered withpre-wetting fluid from the period after the pre-wetting fluid is appliedto the surface under vacuum and the time the wafer is moved to and isimmersed into the plating bath. In some embodiments, the pre-wettinglayer, just before immersion into the plating solution, is thin (e.g.,about 50 to 500 μm thick) and uniform. By the film being thin, theamount of concentration increase or dilution/modification of the platingbath concentrations is kept small, and the film has a minimal delay inadsorption of plating additives to the general plating surface (i.e.,field region). By the film thickness being uniform, the uniformtransition from the state of being covered with a solution ofpre-wetting fluid composition to plating bath solution composition ispossible and much more easily controllable.

Another consideration is that when transferring the wafer from apre-wetting station to a plating station, the features are filled andthe general surface is coated with the pre-wetting fluid. During theensuing time between the initial exposure of the surface to the platingsolution and the initiation of plating, various unfavorable reactionswith the constituents of the pre-wetting fluid, either alone, or incombination with gasses, coming from the atmosphere, may occur. Bydegassing the pre-wetting fluids (e.g., by using a degasser, asdescribed herein), those reactions that involve dissolved gasses may bereduced or eliminated. Still, if and when the liquid surface layer ofthe pre-wetted wafer is exposed to air, gas re-adsorption into thedegassed pre-wetting fluid will occur (e.g., after 15 seconds or more),and may lead to deleterious corrosion or other effects. Alternatively,with an appropriate choice of components included in the composition ofthe pre-wetting fluid and/or timely/rapid wafer transfer to the platingcell, such reaction and effects may be reduced or altogether avoided.

In general, a reaction between the pre-wetting fluid and the seed layeron a wafer results from the existence of a chemical driving force (i.e.,a negative free energy for reaction) with moderate activation energies.Eliminating the driving force, or inhibiting the kinetics, can forestalla deleterious reaction. The reactions involve a combination of one ormore solvents (e.g., water, alcohols, carbonates, or ketones),pre-wetting fluid solutes (e.g., acids, inorganic salts, organicelectrolytic or neutral plating additive species), and dissolved gasses.

An example of a particularly deleterious reaction is the corrosionreaction of the metal seed layer. The seed corrosion rate will depend,for example, on parameters such as the pre-wetted wafer transfer time,the temperatures of the pre-wetting fluid and plating solution bath, thechoice of pre-wetting solvent(s), the pH of the pre-wetting fluid, theparticular dissolved constituents in the pre-wetting fluid, and anyspatial and time-varying distribution or redistribution (i.e.,concentration difference due to the diffusion into or out of thefeatures) during the initial immersion of the wafer into the platingsolution bath. These different reactions are described herein.

Any electrolytic reaction for the corrosion of a metal can berepresented as two half-reactions, coupled by the transfer of anelectron in the metal. For example, the reduction of oxygen or otheroxidizing agent in the solvent (the element that is reduced) coupleswith the oxidation of copper metal. The reaction of copper metal withoxygen occurs in two steps, to cuprous ion, and depending on the solventenvironment, presence of complexing agents, and pH, to cupric ion.

Cu→Cu⁺ +e−→Cu⁺² +e−  (8)

The reduction reactions of oxygen, written for either acid or alkalineconditions, are

O₂+4H⁺+4e ⁺→H₂O  (9a)

O₂+2H₂O+4e ⁺→4OH  (9b)

Using an oxygen free pre-wetting fluid prevents reaction 9a or 9b fromoccurring altogether, and hence the corrosion of copper is prohibitedfrom this source. Therefore, removal of oxygen from the pre-wettingfluid is desirable in some embodiments. However, if oxygen isreintroduced into the electrolyte from the environment (e.g., during atransfer from the pre-wetting location to the plating solution),reaction 9a or 9b is again allowed to occur. Similarly, if the supply ofprotons is small (e.g., a pH of greater than about 3), reaction 9a willbe reduced.

Referring to an example situation depicted in FIG. 13, a structure 1301in a wafer substrate 1302 consists of a cavity filled with pre-wettingfluid 1303. The feature surface 1305, walls 1306 and bottom 1307 of thestructure are typically coated with a barrier layer (not shown) below anelectroplateable “seed layer” (e.g., copper, 1304). The thickness of themetal along the walls, particularly on the lower wall 1308, is typicallymuch thinner than that on the surface 1305 (and often also thinner thanthat at the feature bottom 1307) due to the nature of the seeddeposition process (e.g., PVD). Initially, a degassed pre-wetting fluid1303 is introduced to the surface under vacuum that contains no bubblesand little or no dissolved gas (e.g., oxygen). However, some gas maysubsequently be introduced into the liquid from the atmosphere duringthe transfer of the wafer, with a near saturated condition created atthe exposed liquid layer surface 1308. With a much shorter diffusiondistance and resistance to arriving at the surface 1305, oxygenreduction reaction 9a will preferentially start to occur there first.While reaction 8 may occur anywhere along the surface, the reaction maypreferentially occur at point on the surface where it is roughest andits effect is most detrimental where the film is thinnest (with apotential for loss of all seeded metal). Also, the metal corrosion halfreaction may occur preferentially at locations where the oxygenreduction reaction is not co-occurring, such as at deep inside thefeature (e.g., at locations 1307 and 1308). The overall reaction iscompleted by having the electrons created by reaction 8 inside thefeature traveling through the metal along the wall to the feature topand field, where they combine with the oxygen via reaction 9a or 9b. Thewalls may be rough on a microscopic scale due to the feature creationmethod (e.g., from a repetitions application of a SF₆ isotropic RIEetch/C₄F₈ passivation sequence known as an Advance Silicon Etch or the“Bosche” process) and/or the deposition process. Rough metal surfacestend to have a higher local electrochemical activity, so corrosion willbe greater at rough metal surfaces than for a uniform smooth surface.These phenomena will increase the driving for metal loss from theserough metal surfaces. See, for example, a discussion of these phenomenain U.S. Pat. No. 6,946,065.

In some embodiments, a substantially non-conductive (i.e., non-ionic andelectrolyte free) solvent can be effectively used for pre-wetting fluidfor a pre-wetting process performed under vacuum. This is despitefactors that otherwise would lead one to avoid the use such a fluid. Onesuch conceptually negative factor is that the conductivity of such apre-wetting fluid is quite small. At the time immediately afterimmersion of a wafer into a plating bath solution, deposition at thebottom of the feature filled with a non-conductive or low conductivitysolvent is expected to be hindered by the inability to support platingbecause of its inability to support ionic current flow. Anotherpotentially detrimental factor is the formation, after entry of a waferinto an electroplating bath, of a potential and the establishment of aninternal corrosion cell due to the different activities of dissolvedmetals at the wafer surface and within the feature. The electrochemicalpotential difference in the solution, between the bottom of the featureand the top of the feature can be expressed by a form of the Nernstequation:

$\begin{matrix}{{\Delta \; V} = {\frac{RT}{n\; F}\ln \frac{C({feature})}{C({surface})}}} & (10)\end{matrix}$

In equation 10, R is the universal gas constant, T is the absolutetemperature, n is the number of electrons for the corrosion reaction, Fis Faraday's constant, and C(feature) and C(surface) are theconcentrations of metal ions at the two locations. A concentration cellis created, with the corrosion driving potential created by a differencein concentrations as given by equation 10. When using a pre-wettingfluid free of dissolved metal ions, the bottom of a feature willencounter a C(feature) concentration that is smaller than the C(surface)for some period of time after immersion into the plating bath containingmetal ions. Therefore, a corrosive potential difference will existbetween location the bottom of a feature and the surface, with thecorrosion potential causing the metal on the walls and bottom of thefeature to preferentially oxidize, release electrons, and complete thecycle by combining with the metal ions from the solution at the surface.

Specifically, the reaction

Cu⁺⁺+2e−→Cu  (11)

will occur at the surface region, and will be coupled with the oxidationreaction

Cu→2e−+Cu⁺⁺  (12)

occurring on the walls and lower surface at the bottom of the feature.To avoid this undesirable process when using this type of pre-wettingfluid, it is important to establish a cathodic (plating) polarization ofthe wafer surface versus the plating solution prior to, or very shortlyafter, immersion of the wafer surface into the plating solution. (SeeU.S. Pat. Nos. 7,211,175, 6,562,204 and 6,551,483, which deal with entrycathodic protection and potentiostatic entry, herein incorporated byreference in relevant part.) This is accomplished by applying a cathodicpotential difference or a small cathodic current between the wafer andthe solution before wafer bath entry. Alternatively, or additionally, insome embodiments, a rapid rinse of the wafer surface with a solutionhaving a relatively low metal ion concentration can be used (e.g., DIwater), followed by a high speed spin or other method to remove thesurface solution. This process reduces the concentration of the metal atthe surface relative to that in the feature, but also removeselectrolyte from the wafer edge, reducing the propensity for thatelectrolyte to be plated on the edge of the wafer and plating apparatuscontacts (e.g., when plating in a closed or seal contact “plating cup”).As another alternative, the metal ion concentration in the pre-wettingsolution can be at least equal to or greater than that of the subsequentplating bath.

Examples of embodiments of pre-wetting fluids of the substantiallynon-conductive class are an electrolyte-free isopropyl alcohol or otherwater soluble non-aqueous solvents (i.e., water-miscible solvents).Other embodiments include alcohols, a dialkylcarbonate,dimethylformamide, and dimethyl sulfoxide. Another embodiment is asolution of water containing a small concentration of thenon-metal-complexing tetramethylammonium sulfate and/ortetramethylammonium hydroxide, in the pH range between about 3.5 and11.5. Still another embodiment is a solution of water containing asurfactant such as the anionic surfactant laurilsulfate (with an alkalimetal cation or tetramethylammonium cation). Pre-wetting fluids thathave a reduced surface tension compared to water, relatively smallconductivities (e.g., compared to acids or strong bases), and arenon-copper complexing are used in some embodiments.

In some embodiments of an electroplating process 1200 for electroplatinga layer of metal on a wafer substrate shown in FIG. 12, a wafersubstrate having an exposed metal layer on at least a portion of itssurface is provided to a pre-wetting process chamber (1205). The waferis then contacted with a pre-wetting fluid to form a layer ofpre-wetting fluid on the wafer substrate (1210). The pre-wetting fluidincludes a water-miscible solvent. The water miscible solvent may be analcohol, ketone, dimethylcarbonate, diethylcarbonate, dimethylsulfoxide, or dimethyl formamide. The pre-wetted wafer is then contactedwith a plating solution that includes metal ions to electroplate a layerof metal on the wafer substrate (1215). In some embodiments, the platingsolution includes copper ions to electroplate a layer of copper on thewafer substrate.

To elaborate, because the half oxidation reactions (e.g., reactions 8and 9) must complete an electrical circuit and therefore pass ionicelectrical current between the two locations where oxidation andreduction reactions are occurring, in some embodiments it isadvantageous to use a pre-wetting fluid that has a small ionicconductivity, such as a low conductivity solvent that itself will notreact directly with copper, or a solvent substantially free of ionicallydisassociated and conductive ions (i.e., dissolved acid, bases, andsalts). Many water soluble solute free solvents, such as DI water,isopropyl alcohol, ethylene glycol, propylene glycol, propylenecarbonate, etc., have high electronic resistances when free of solutes,and the solubility of the cupric or cuprous ion in a neutral pH solventin the same are also generally very small. Because of these factors,corrosion of the metal in these solvents can only occur by the directoxidation with dissolved oxygen, generally a very slow process atambient temperature and oxygen concentrations:

Cu+½O₂→Cu₂O  (13)

Therefore, performing pre-wetting using ionic-solute-free solvents(i.e., a non-electrolytic solution) such as water or DI water is oneembodiment, and using deoxygenated deionized solvents such asdeoxygenated DI water is another embodiment (to avoid reaction 13). Insome embodiments, the plating solution is also deoxygenated/degassedprior to and during contact/exposure to the wafer surface, and apotential or current is applied to the wafer prior to entry,establishing a voltage greater than that given by equation 10, therebypreventing reaction 12 within the feature from occurring. With respectto preventing side wall corrosion, pre-wetting fluids that containnon-ionic dissolved species (e.g., non-ionic surfactants orswitter-ionic surfactants, added to lower surface tension, or organicmaterials added to react with oxygen) are useful in some embodimentsover highly conductive ionic solutes such as acids and bases. This isbecause of the generally lower solution specific conductance and ioncurrent coupling of the oxidation and corrosion half reactions. Anexception to this is the addition of surface adsorbing electrochemicallyactive non-ionic materials (e.g., a non-ionic leveler compounds). Afurther example of an unfavorable pre-wetting fluid combination forcopper plating, in some embodiments, is polyethylene glycol orpolyethylene/polypropylene oxide copolymer (which are known to act asplating “suppressors”) at all but very low concentrations combined withsmall amount of dissolved halide (e.g. chloride) ions. A suppressorwithout the adsorption and electrochemical activity enhancing halidesappears to be unfavorable, in some embodiments, unless at very lowconcentrations.

In one experiment performed according to an embodiment described herein,a 60 μm deep/10 μm wide TSV via structure having a 8000 Å copper seedlayer was electroplated with copper. The feature was pre-wetted withdeoxygenated, deionized water. After the wafer was exposed to theatmosphere for 5 minutes, it was then transferred to a plating cell,followed by immersion into the plating solution. The plating solutionwas a deoxygenated plating bath sold under the trademark DVF 200™ byEnthone Inc. with added components (DVF 200™ is a copper methanesulfonate/methane sulfonic acid plating solution to which accelerators,suppressors, and leveler additives, and 50 ppm chloride ions, wereadded). The filling characteristics of multiple individual featuresacross multiple runs using this method showed complete void-free featurefilling in almost all cases. In some experiments, the wafers werecathodically polarized prior to entry into the plating solution. Thisresult shows the process robustness for creating void-free bottom-upfilling using a combination of a degassed DI water pre-wetting processperformed under vacuum, followed by potentiostatic entry into a copperplating solution.

In other embodiments, substantially non-conductive pre-wetting fluidsthat contain some dissolved compounds other than metals (e.g., eitherelectrolytic or non-ionic, organic or inorganic, added, for example, inrelatively small quantities to reduce surface tensions and aid inwetting), but are substantially free of materials that areelectrochemically active and/or are considered plating bath additives,are used. For example, in some embodiments, a pre-wetting fluid that issubstantially free from any accelerator/brighteners or levelers (thatmight typically be found in a subsequently used plating bath) is usedrather than those that do contain such electrochemically active agents.

In one experiment performed according to an embodiment described herein,a wafer with a 60 μm deep/10 μm wide TSV via structure having a 8000 Åcopper seed layer was electroplated with copper. The feature waspre-wetted with a pre-wetting fluid containing copper methane sulfonicacid (copper salt, 80 g/L copper ion), 20 g/L methane sulfonic acid, 50ppm chloride ion, and 3 or 12 ppm of the copper plating acceleratordimercapto-propane sulfonic acid (SPS). After pre-wetting, the waferswere exposed to the atmosphere for about 1 minute, and then transferredto a plating cell, followed by immersion into the plating solution. Theplating solution was a deoxygenated plating bath sold under thetrademark DVF 200™ by Enthone Inc. with additive components. Copper wasthen plated onto the wafer. In both cases (i.e., pre-wetted with asolution containing 3 ppm of dimercapto-propane sulfonic acid and 12 ppmof dimercapto-propane sulfonic acid), sidewall voids were formed.

In some embodiments, the pre-wetting fluid includes water and a coppersalt. This helps to avoid corrosion of a seed layer due to setting upthe electrochemical difference discussed in relation to equation 10. Incertain embodiments, the copper salt is at a concentration of at leastabout 50% of a saturation limit. In certain embodiments, the copper saltis copper sulfate, a copper alkylsupphonate, and mixtures thereof. Incertain embodiments, the copper salt is at a concentration greater thanabout 20 g/L of copper. In some embodiments, after pre-wetting the wafersubstrate with a pre-wetting fluid that includes water and a coppersalt, the pre-wetted wafer substrate is electroplated with copper with acopper-containing plating solution; the pre-wetting fluid contains acopper salt at a copper concentration that is the same or higher thanthe copper concentration in the plating solution. In some embodiments,the copper concentration in the pre-wetting fluid is at least about 25%greater than the copper concentration in the plating solution. In otherembodiments, the pre-wetting fluid consists essentially of water and acopper salt.

In some embodiments, a pre-wetting fluid having the same or a verysimilar composition to that of the plating solution (i.e., a solutionhaving the same metal salts and/or the same metal ion concentrations,the same acids and/or the same acid concentration(s), the same halidesand/or the same concentrations of halides, and the same additives and/orthe same concentrations of additives) is used. In embodiments when thepre-wetting fluid and the plating solution have the same composition, alayer of metal may be plated on the wafer substrate in the same chamberas used for pre-wetting. However, when a seed layer is marginal (e.g.,rough and thin within the feature), such a pre-wetting fluid (i.e., thesame or very similar to the plating solution) may be susceptible tofeature fill voiding due to seed corrosion from the pre-wetting fluid.The feature rate of filling may also be improved by using a differentsolution for pre-wetting than that of the plating bath, as describedherein.

In one experiment performed according to an embodiment described herein,a 60 m deep/10 m wide TSV via structure having a 8000 Å copper seedlayer was electroplated with copper. The feature was initiallypre-wetted with a plating solution (i.e., the pre-wetting fluid had thesame composition as the plating solution). The feature/wafer waspre-wetted via a pre-wetting process performed under vacuum describedherein with commercially available deoxygenated plating bath sold underthe trademark DVF 200™ by Enthone Inc., with plating additive components(i.e., the plating additive components used with DVF 200^(m) in theother experiments described herein). The surface was exposed to thedeoxygenated plating bath and was then exposed to the atmosphere foreither 1 or 3 minutes between release of vacuum and thetransfer/immersion into the plating bath and initiation of metaldeposition. The wafers were cathodically polarized immediately uponentry into the plating solution. In one case where the surface wasexposed to the atmosphere for 1 minute, the feature was filled withmetal and void free, with no evidence of side wall corrosion. However,features from the same wafer show that some features are not filled,typically with one side of the feature having an irregularly shapedvoid. This is generally believed to be associated with the loss of seedmetal at the side of the feature. For a wafer prepared and processed inexactly the same fashion, except that there was 3 minutes exposure toatmospheric between release of vacuum and the initiation of plating, thefeature filling was grossly incomplete. In many cases, the entire bottomof the feature was unplated. A similar trend (i.e., transition from voidfree to significant side wall voiding) also occurs for a fixedatmospheric exposure time, but with decreasing seed layer thickness.Therefore, the use of the plating solution as a pre-treatment solutionis less than optimal in some embodiments because of its significantsensitivity to incomplete feature filling due to side wall corrosion.Particularly in situations where the seed layer thickness is quite thin,the number of side wall void-type defects increased markedly in bothsituations, indicating a narrow tolerance of seed layers to thispre-wetting fluid.

Referring back to reactions 8 and 9, the metal ion created by thecoupling of reaction 8 with reaction 9a or 9b must be able to passelectrical (ionic) current back to the surface though the fluid, sohaving a solution of substantial conductivity is an unfavorablepre-wetting fluid property, in some embodiments. This is in contrast thesubstantial conductivity that is generally desired in a platingsolution, where conductivity is tailored so as to minimize voltage dropsin the solution and within the feature to facilitate the depositionprocess. Of particular interest is the high ionic mobility of protons,which is the highest of any cation. This property tends to impart veryhigh conductivities to acid solutions of given molarities. Therefore, asa general rule, pre-wetting fluids having highly dissociated acids ofhigh concentrations (e.g., those that generate a pH of less than about 2or create greater than about 0.01 molar of free protons) are not favoredin some embodiments because they facilitate corrosion reactions due totheir high conductivities. Under such acidic conditions, the metal atthe bottom of the feature's wall 1308 (FIG. 13) is subject tounfavorable conditions and could potentially result in the areacorroding and producing a side wall without an electroplateable seedlayer.

As a primary consideration, it is desirable to avoid thisinside-the-feature corrosion and feature filling defects such asvoiding. The combination of the high conductivity, acidity, andpotentially unfavorable adsorption and reaction of additives andhalide(s) with the metal can lead to, for example, feature sidewallcorrosion and filling defects, as well as inhibiting or delaying theestablishment of a favorable distribution of the additives on thevarious surfaces required for optimal feature filling rates or void freefilling. Because the metal on the side wall can be thin and oxidizedprior to the pre-wetting fluid exposure, a corrosion reaction involvingacid or other components can lead to the loss of all the platable metal,leaving a non-plateable metal such as the copper diffusion barriertantalum or tantalum nitride with an exposed oxide layer thereupon. Longunpolarized (cathodically protected) exposure of the surfaces toinappropriate component mixtures of the pre-wetting fluid may thereforelead to poor feature filling. Contrary to using a strongly acidicelectrolyte (pH of less than about 2), using more neutral or nearneutral pre-wetting fluid can limit the supply of protons for reaction9a, reducing the corrosion rate, reducing the defectivity, and generallyimproving the reliability and success of the overall pre-wetting. Notethat pre-wetting fluids of this description would generally not beoptimal or acceptable for copper metal deposition, but they are favoredfor pre-wetting, in some embodiments. Solutions in the pH ranges ofabout 2 to 12, free of dissolved metal ion complexing anions, do notallow reactions such as 8 and 9 to occur at appreciable rates.

In some embodiments, the pre-wetting fluid includes deionized water, anacid, and a copper salt, with the pre-wetting fluid pH not lower thanabout 2. In further embodiments, the pH of such a pre-wetting fluid isbetween about 2 and 4. The acid in such embodiments may be sulfuricacid, an alkylsuphonic acid, and mixtures of these acids. Thepre-wetting fluid may also include less than about 2 g/L or sulfuricacid or methane sulfonic acid in some such embodiments. In otherembodiments, the pre-wetting fluid consists essentially of water, anacid, and a copper salt, with the pre-wetting fluid pH greater thanabout 2. In yet other embodiments, the pre-wetting fluid includes waterand an acid, and the pre-wetting fluid has a pH of greater than about 2.

According to the various published pH/potential stability diagrams andcalculations (also known as Pourbaix diagrams), exposure of copper metalto a non-complexing electrolytic solution of about pH 3 or greater andfurther having an oxidizing source (such as dissolved oxygen) isexpected to form a metal surface oxide. The formation of the oxideinstead of a dissolved metal salt of copper may inhibit furtheroxidation. It is thermodynamically favored for cuprous ions formed atthe interface to react with water or hydroxide directly to form cuprousoxide or hydroxide (rather than form dissolved cuprous or cupric salts).

2Cu+4OH⁻→Cu₂O+H₂O+4e ⁻  (15a)

Cu+2OH⁻→Cu(OH)₂+2e ⁻→CuO+H₂O+2e ⁻  (15b)

At a very high pH, the hydroxide of copper is slightly soluble, so thiscondition is slightly unfavorable from this prospective. The coupling ofthe copper oxidation half reaction with the oxygen reduction reactionmay be reduced in a neutral solution, and so pre-wetting fluids free ofcopper complexing agents in a pH range of from about 2 to 12, moreparticularly between about 3.5 to 10.5, are useful classes ofpre-wetting fluids for use under vacuum. Pre-wetting fluids of thisclass are solutions that may contain some dissolved compounds (bothelectrolytic and non-ionic, organic or inorganic, for example, to reducesurface tensions and aid in wetting), but are substantially free ofmaterials that electrochemically alter the plating of the metal andactive and/or are considered plating bath additives. The presence of acopper complexing agent also changes conditions, allowing the formationof a complex instead of the passivating oxide/hydroxides; if oxygen ispresent, unfavorable high rate corrosion is expected in metal complexingagent containing solutions having a dissolved oxidizer. Some materialsthat are typically bath additives can form metal complexes, such asmercapto-group containing brighteners/accelerators (e.g.,mercapto-propanesulfonic acid, di-mercaptopropane sulfonic acid, etc.)and various nitrogen group containing levelers (e.g., diazine black andJanus Green B). For example, a pre-wetting fluid that does not containany brighteners or levelers that might typically be found in asubsequently used plating bath may avoid related pre-wetting seed metalcorrosion. Suppressors such as polyethers (e.g., polyethylene glycol,polypropylene oxide, etc.) or metal ion complexing agents by themselvesare not particularly corrosive, and since they tend to reduce thesurface tension as wetting agents, can be added when high rate fill isnot a primary concern. However, the addition of suppressor incombination with chloride ions, generally considered a necessaryco-constituent to achieve suppressor electrochemical activity, is notfavored in some embodiments.

In some embodiments, the pre-wetting fluid may aid in removing the oxidesurface. In some embodiments of an electroplating process 1200 forelectroplating a layer of metal on a wafer substrate shown in FIG. 12, awafer substrate having an exposed metal layer on at least a portion ofits surface is provided to a pre-wetting process chamber (1205). Thepressure in the pre-wetting process chamber is then reduced to asubatmospheric pressure (not shown). The wafer is then contacted with apre-wetting fluid to form a layer of pre-wetting fluid on the wafersubstrate (1210). In one embodiment, the pre-wetting fluid includes anacid to at least partially remove surface oxide from the seed layer andthe pre-wetting fluid has a pH of between about 2 to 6. The pre-wettedwafer is then contacted with a plating solution that includes metal ionsto electroplate a layer of metal on the wafer substrate (1215). Theplating solution has a pH of between about 2 to 6 and the platingsolution and the pre-wetting fluid have different compositions.

In other embodiments, the pre-wetting fluid may aid in transforming ametal oxide laden surface to a metallic surface (e.g., cuprous or cupricoxide—see reactions 6 and 7 and the associated discussion) or inremoving the oxide surface. In some embodiments of an electroplatingprocess 1200 for electroplating a layer of metal on a wafer substrateshown in FIG. 12, a wafer substrate having an exposed metal layer on atleast a portion of its surface is provided to a pre-wetting processchamber (1205). The wafer is then contacted with a pre-wetting fluid toform a layer of pre-wetting fluid on the wafer substrate (1210). In oneembodiment, the pre-wetting fluid includes a small amount of a reducingagent to at least partially reduce surface oxide on the seed layer. Inanother embodiment, the pre-wetting fluid includes a metal complexingagent to at least partially remove surface oxide on the exposed metallayer and the pre-wetting fluid has a pH of between about 4 to 12. Thepre-wetted wafer is then contacted with a plating solution that includesmetal ions to electroplate a layer of metal on the wafer substrate(1215).

In some embodiments, the plating solution includes copper ions toelectroplate a layer of copper on the wafer substrate. In suchembodiments, the exposed metal layer on the wafer substrate is generallycopper or a copper alloy. Examples of reducing agents for copper includeformaldehyde, glycolic acid (and salts thereof), and dimethylamineborane. When the exposed metal layer is copper, the pre-wetting fluidincludes, in some embodiments, a copper complexing agent to at leastpartially remove surface oxide on the exposed copper layer and thepre-wetting fluid has a pH of between about 4 to 12.

In general, small concentrations (e.g., parts per million, typically 10to 100 ppm) of halide ions such as chloride or bromide are present andoften critical in many plating bath solutions. Halides are also wellknown corrosion agents. It is generally known that a solution containinghalides will corrode a surface faster than an identical solution (i.e.,consistent pH and ionic strength) without halides. Because they arecritical in successful plating and their concentrations are low, onemight assume that not having them present in the pre-wetting fluid wouldinhibit their uniform exposure to the features internal surfaces andthereby have a detrimental effect to the feature filling process. Insome embodiments, however, it is useful to not include or add even thesevery low levels of halides to the pre-wetting fluids. In someembodiments, the pre-wetting fluid is substantially free from halides.Even with low, parts per million levels of halides, alone or incombination with other plating bath additives, dramatic increases in thecorrosion rate of the metal on side wall features have been observed.While not wanting to be held to any particularly theory, corrosion ofthe metal, as a whole, is perhaps catalyzed or stabilized by theformation of cuprous halide reactants.

In one TSV feature filling experiment, similar to the other experimentsdescribed herein, performed according to an embodiment described herein,the pre-wetting fluid contained 100 g/L copper methane sulfonic acid, 16g/L methane sulfonic acid, and either no chloride or 50 ppm of chlorideions. The copper plating was then performed using the same solution andprocesses as described for the other experiments described herein. Sidewall seed layer corrosion was greatly exacerbated by even smallquantities of halide ions in the pre-wetting fluid.

Plating bath suppressors include polyethylene glycol (PEG),polypropylene glycol (PPG), polyethylene oxide (PEO), polypropyleneoxide (PPO), and various copolymers of these monomers. A suppressor isused to suppress copper plating outside the features on a wafer,allowing for copper deposition inside such features. They are also goodsurface tension reducing agents (surfactants), and hence, might beconsidered to be useful component in the pre-wetting fluid. As notedherein, the suppressive plating characteristics of these compounds aregenerally derived in combination with halides, and the presence ofhalides can cause feature side wall corrosion. In some embodiments, thepre-wetting fluid is substantially free from halides, platingaccelerators, and plating levelers, and includes a plating suppressor ata small concentration (e.g., typically less than about 15 ppm).

An experiment was performed using pre-wetting fluid containing 100 g/Lcopper methane sulphonate, 16 g/L methane sulfonic acid (sometimesreferred to as virgin makeup solution or “VMS”), no chloride ions, andvarious amount of an 8000 molecular weight polyethylene glycol todetermine the effects of suppressors on a pre-wetting fluid. Side wallcorrosion was generally not observed in these chloride-free pre-wettingfluids. Feature filling, however, was significantly impacted as theconcentration of the suppressor reached somewhere between about 5 partsand 25 parts per million. The fill characteristics transitioned from abottom-up filling at from about 0 to 5 ppm of suppressor to a bottomvoid at about 25 ppm suppressor. At about 50 ppm of suppressor andabove, the plating was largely conformal. Thus, the use of above about15 ppm of a suppressor is undesirable from a feature filling prospectivein some embodiments.

In some embodiments of an electroplating process 1200 for electroplatinga layer of copper on a wafer substrate shown in FIG. 12, a wafersubstrate having an exposed metal layer on at least a portion of itssurface is provided to a pre-wetting process chamber (1205). The waferis then contacted with a pre-wetting fluid to form a layer ofpre-wetting fluid on the wafer substrate (1210). The pre-wetting fluidincludes water and copper ions, and is substantially free of platingadditives. In this embodiment, the plating solution includes platingadditives. The concentration of copper ions in the pre-wetting fluid isgreater than the concentration of copper ions in the plating solution.In some embodiments, the pre-wetting fluid is substantially free ofadditives, including halides, accelerators, and levelers, andcombinations thereof. In some embodiments, the pre-wetting fluidincludes polyethylene oxide at a concentration of less than about 15ppm. In some embodiments, plating solution additives include halides,accelerators, suppressors, and combinations thereof. The pre-wettedwafer is then contacted with a plating solution that includes copperions to electroplate a layer of copper on the wafer substrate (1215).

In addition to the possibility of a particular pre-wetting fluid eitheraiding or hindering the feature filling process (e.g., the avoidance ofside wall corrosion or creating a conformal type filling behavior),there is also a relationship between the composition of the pre-wettingfluid and the rate of feature filling. In experiments performed tocompare the rates of feature filling, the plating bath solutioncomposition and plating currents versus time were fixed and the amountof feature filling at the end of the process was monitored. Theexperiments showed that the choice of pre-wetting fluid can have adramatic effect on the feature filling rate and time, sometimesincreasing the filling rate and reducing the filling time by a factor oftwo or more.

While not wanting to be held to any particular explanation or model forthis effect, it is believed that having a conductive electrolyte (e.g.,as opposed to, for example, DI water) that principally containssignificant amount of the metal salt is necessary to rapidly initiateand sustain plating at the bottom of a feature. In some embodiments, thepre-wetting fluid should not include some or all of the plating bathadditives which are necessary in the field to suppress plating thereupon(i.e., levelers and suppressor). In some embodiments, the pre-wettingfluid is substantially free from plating levelers. The addition ofaccelerator to the pre-wetting fluid is useful in some embodiments, butaccelerators (such as dimercaptopropane sulfonic acid, SPS) tend tobifurcate and form a very strongly adsorbing accelerator monomer (e.g.,mercaptopropane sulfonic acid, MPS) simply by exposure of the surface tothe pre-wetting fluid. This bifurcation of accelerator may besufficiently fast that, prior to initiating plating, the entire surfaceis saturated with the accelerator adsorbate. Therefore, upon startingthe subsequent plating, the field and upper side walls may require theaccelerator to be removed or deactivated in order to cause the currentto be driven into the intricacies of the features. In contrast, apre-wetting fluid that contains metal ions, but no accelerator or otheradditives, will plate at a high rate upon entry in the bath until asuppressing additive become surface-active. In some embodiments, thepre-wetting fluid is substantially free from one or more of halides,plating accelerators, and plating levelers.

The smaller accelerator can diffuse quickly from the plating bath intothe mouth and lower regions of a feature, while the suppressor andleveler molecules will diffuse slower and initially act primarily on theupper side walls of the feature, hence creating polarization contact anddriving the current into the feature. The accelerator molecule primarilyserves the purpose of deactivating, removing, or preventing thepolarization developed by the suppressor/halide combination. Anaccelerator in itself is only a weak polarizing agent with respect to anadditive free solution that is free of the polarizing suppressor/halidecombination. While some halides, such as chloride, may be relativelysmall and diffuse with similar alacrity to the accelerator molecule,without either suppressor and leveler present at the feature bottom, thesurface kinetics will still be fast and the plating resistance very low(halides alone are generally non-polarizing, and in fact some literaturesays they are depolarizing, by themselves). Furthermore, the ability tohave the electrochemical conversion of SPS to the strongly adsorbing MPSaccelerator molecule occur for some duration of time in an localenvironment free of suppressor (i.e., before any suppressor can arrivesets up an “inertial” plating condition that tend to reduce thepropensity for the suppressor later absorb) avoids polarization andincreases the relative plating to the bottom of the feature. Incontrast, at the upper walls and field of the feature, very shortlyafter exposure to the plating solution, suppressor is adsorbed, andleveler competes with the adsorption of accelerator to remove thepolarization there, so polarization develops there very rapidly. As timeprogresses after immersion into the plating bath, the small moleculessuch as the accelerator and halide will diffuse rapidly into thefeature, but the larger levelers and much larger suppressor will diffusemuch more slowly, delaying their inhibitive effect inside and allowingfor a fast fill.

Depending on the feature size, quality of seeding inside the feature,various processing costs, and other goals, one or more pre-wettingfluids may be favored over another, in certain embodiments. Tables 1 to4 are based a large number of feature filling experiments andobservations, similar to those described herein, that qualitativelycategorized and compared the propensity of feature corrosion andenhancement/retardation of feature filling (filling rate) for a numberof different pre-wetting fluid combinations. The table term “EXCELLENT”indicates a generally highly favorable result (e.g., little evidence ofseed corrosion or enhanced or otherwise high feature filling rate). Thetable term “GOOD” indicates a potentially acceptable result, though onethat may not be optimal in all cases (e.g., depending on seed quality,plating baths, etc). The table term “FAIR” includes performances thatare typically quite marginal or may be unreliable, and may often lead tonegative or poor results. Finally, the table term “POOR” indicatesalmost invariably unacceptable seed corrosion or a significantly altered(e.g., conformal filling) or diminished filling rate behavior.

Results for different concentrations of acids are given. Results onmetals of either sulfate or methane sulfonic acids are also given,although the difference between the two was generally found to beminimal. In tables where the labels “sulfuric or methane sulfonic” acidare given, the additional component creates a mixture of the two typesof acids (not just more acid of the same chemical). For example, inTable 2, all of the pre-wetting fluids included greater than 2 g/L oreither sulfuric acid or methane sulfonic acid. The pre-wetting fluid inrow 1 [i.e., >2 g/L Methane Sulfonic (or Sulfuric) Acid] contained BOTH2 g/L of sulfuric acid AND 2 g/L of methane sulfonic acid. Thepre-wetting fluid in row 2 [i.e., <2 g/L Methane Sulfonic (or Sulfuric)Acid] contained EITHER 2 g/L of sulfuric acid OR 2 g/L of methanesulfonic acid.

Sulfuric acid has about the same molecular weight as methane sulfonicacid, so the concentrations are about the same in the two cases, but itis recognized that sulfuric acid is both diprotic and has a differentdissociation constant (H₂SO₄: MW=98, pKa₁=3.0, pKa₂=2; CH₃SO₃H: MW=96,pKa=1.9), so the pH of a solution containing the same amount of sulfuricacid will be lower. Finally, the concentrations of the copper solutionsfrom different salts are in grams per liter of cupric ions (Cu⁺⁺), notof the anhydrous salt or hydrated salt.

From these tables a number general trends of good pre-wetting fluids canbe identified, specifically those containing little or no acid (pH of 2or greater), moderate to high metal ion concentrations, little or nohalides (<10 ppm), less than about 15 ppm PEG like suppressors, andneither leveler or accelerator plating additive. A solution containingfrom 20 to 100 g/L metal ion with no other components other than solvent(water) and a small concentration of a surfactant (or no surfactant) isan example of a good pre-wetting fluid composition.

TABLE 1 Pre-wetting fluid of DI water and one other component. FeatureFeature Sidewall Bottom Up Material Corrosion Filling DI water and OneOther component Susceptabilty Rate DI Water Only Excellent Fair-Good >2g/L Sulfuric Acid Poor Good >2 g/L Methane Sulfonic Acid Poor Good <2g/L Sulfuric Acid Fair-Good Fair-Good <2 g/L Methane Sulfonic AcidFair-Good Fair-Good <20 g/L Copper Sulfate Excellent Good <20 g/L CopperMethane Sulfonate Excellent Good >20 g/L Copper Sulfate ExcellentExcellent >20 g/L Copper Methane Sulfonate Excellent Excellent >10 ppmChloride/Bromide Poor Poor >15 ppm Suppressor Good Fair Accelerator FairPoor Leveler Fair Poor

TABLE 2 Pre-wetting fluid of DI water, greater than 2 g/L sulfuric acidor methane sulfonic acid, and one other component. Material FeatureFeature DI water, >2 g/L Sulfuric Acid or Sidewall Bottom Up MethaneSulfonic Acid, and One Corrosion Filling Other component SusceptabiltyRate >2 g/L Methane Sulfonic (or Sulfuric) Acid Poor Good <2 g/L MethaneSulfonic (or Sulfuric) Acid Poor Good <20 g/L Copper Sulfate Fair Good<20 g/L Copper Methane Sulfonate Fair Good >20 g/L Copper Sulfate FairExcellent >20 g/L Copper Methane Sulfonate Fair Excellent >10 ppmChloride/Bromide Poor Poor >15 ppm Suppressor Good Fair Accelerator PoorPoor Leveler Poor Poor

TABLE 3 Pre-wetting fluid of DI water, less than 2 g/L sulfuric acid ormethane sulfonic acid, and one other component. Material Feature FeatureDI water, <2 g/L Sulfuric Acid or Sidewall Bottom Up Methane SulfonicAcid and One Corrosion Filling Other Component Susceptabilty Rate >2 g/LMethane Sulfonic (or Sulfuric) Acid Poor Good <2 g/L Methane Sulfonic(or Sulfuric) Acid Fair-Good Good <20 g/L Copper Methane Sulfonate GoodGood <20 g/L Copper Sulfate Good Good >20 g/L Copper Sulfate ExcellentExcellent >20 g/L Copper Methane Sulfonate Excellent Excellent >10 ppmChloride/Bromide Poor Poor >15 ppm Suppressor Good Fair Accelerator FairPoor Leveler Fair Poor

TABLE 4 Pre-wetting fluid of DI water, less than 2 g/L sulfuric acid ormethane sulfonic acid, greater than 20 g/L copper sulfate, and one othercomponent. Material Feature Feature DI water, <2 g/L Sulfuric AcidSidewall Bottom Up (or Methane Sulfonic Acid), >20 g/L Corrosion FillingCopper Sulfate, and One Other Component Susceptabilty Rate >2 g/LSulfuric (or Methane Sulfonic) Acid Fair Excellent <2 g/L Sulfuric Acid(or Methane Sulfonic) Good Excellent Acid <20 g/L Copper MethaneSulfonate Excellent Excellent >20 g/L Copper Methane Sulfonate ExcellentExcellent >10 ppm Chloride Poor Poor >15 ppm Suppressor Excellent FairAccelerator Fair Poor Leveler Fair Poor

CONCLUSION

Although the foregoing apparatus designs and methods have been describedin some detail for purposes of clarity of understanding, it will beapparent that certain changes and modifications may be practiced withinthe scope of the appended claims. It should be noted that there are manyalternative ways of implementing both the process and compositionsdescribed herein. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and embodiments are notto be limited to the details given herein.

1. A method of electroplating a layer of metal on a wafer substrate, themethod comprising: (a) providing the wafer substrate having an exposedmetal layer on at least a portion of its surface to a pre-wettingprocess chamber; (b) contacting the wafer substrate with a pre-wettingfluid comprising a water-miscible solvent to form a layer of pre-wettingfluid on the wafer substrate; (c) contacting the pre-wetted wafersubstrate with a plating solution comprising metal ions to electroplatea layer of metal on the wafer substrate.
 2. The method of claim 1,wherein the water-miscible solvent is selected from the group consistingof an alcohol, dimethylcarbonate, diethylcarbonate, dimethyl sulfoxide,and dimethyl formamide.
 3. The method of claim 1, wherein the platingsolution comprises copper ions to electroplate a layer of copper on thewafer substrate.
 4. A method of electroplating a layer of metal on awafer substrate, the method comprising: (a) providing the wafersubstrate having an exposed metal layer on at least a portion of itssurface to a pre-wetting process chamber; (b) reducing pressure in thepre-wetting process chamber to a subatmospheric pressure; (c) contactingthe wafer substrate, at a subatmospheric pressure, with a pre-wettingfluid comprising an acid to at least partially remove surface oxide fromthe exposed metal layer and to form a layer of pre-wetting fluid on thewafer substrate, wherein the pre-wetting fluid has a pH of between about2 to 6; (d) contacting the pre-wetted wafer substrate with a platingsolution comprising metal ions to electroplate a layer of metal on thewafer substrate, wherein the plating solution has a pH of between about2 to 6, wherein the plating solution and the pre-wetting fluid havedifferent compositions.
 5. The method of claim 4, wherein the platingsolution comprises copper ions to electroplate a layer of copper on thewafer substrate.
 6. A method of electroplating a layer of metal on awafer substrate, the method comprising: (a) providing the wafersubstrate having an exposed metal layer on at least a portion of itssurface to a pre-wetting process chamber; (b) contacting the wafersubstrate with a pre-wetting fluid comprising a reducing agent to atleast partially reduce surface oxide on the exposed metal layer and toform a layer of pre-wetting fluid on the wafer substrate; (c) contactingthe pre-wetted wafer substrate with a plating solution comprising metalions to electroplate a layer of metal on the wafer substrate.
 7. Themethod of claim 6, wherein the plating solution comprises copper ions toelectroplate a layer of copper on the wafer substrate. 8-9. (canceled)